refactor: use string buffer instead of format!

[ptondereau]

Two hot-path functions were allocating throwaway strings on every iteration in a loop. Now they reuse a single buffer instead. The benchmark confirms a 10-15% speedup on every incoming request that hits query parsing or variable replacement.

Criterion results

  Compiling redirectionio v3.0.0-beta.2 (/home/ptondereau/Code/libredirectionio)
    Finished `bench` profile [optimized] target(s) in 8.49s
     Running benches/string_alloc_benchmark.rs (target/release/deps/string_alloc_benchmark-e91835232e194c03)
Benchmarking StaticOrDynamic::replace/2
Benchmarking StaticOrDynamic::replace/2: Warming up for 3.0000 s
Benchmarking StaticOrDynamic::replace/2: Collecting 100 samples in estimated 5.0002 s (22M iterations)
Benchmarking StaticOrDynamic::replace/2: Analyzing
StaticOrDynamic::replace/2
                        time:   [209.35 ns 210.56 ns 211.74 ns]
                        change: [−15.206% −14.599% −14.034%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  1 (1.00%) low severe
  3 (3.00%) low mild
  4 (4.00%) high mild
  1 (1.00%) high severe
Benchmarking StaticOrDynamic::replace/5
Benchmarking StaticOrDynamic::replace/5: Warming up for 3.0000 s
Benchmarking StaticOrDynamic::replace/5: Collecting 100 samples in estimated 5.0007 s (7.9M iterations)
Benchmarking StaticOrDynamic::replace/5: Analyzing
StaticOrDynamic::replace/5
                        time:   [674.04 ns 679.22 ns 685.53 ns]
                        change: [−11.100% −10.549% −9.9036%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  1 (1.00%) low severe
  2 (2.00%) high mild
  3 (3.00%) high severe
Benchmarking StaticOrDynamic::replace/10
Benchmarking StaticOrDynamic::replace/10: Warming up for 3.0000 s
Benchmarking StaticOrDynamic::replace/10: Collecting 100 samples in estimated 5.0058 s (3.4M iterations)
Benchmarking StaticOrDynamic::replace/10: Analyzing
StaticOrDynamic::replace/10
                        time:   [1.4798 µs 1.4842 µs 1.4887 µs]
                        change: [−19.648% −19.091% −18.586%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  1 (1.00%) low mild
  4 (4.00%) high mild
  1 (1.00%) high severe
Benchmarking StaticOrDynamic::replace/20
Benchmarking StaticOrDynamic::replace/20: Warming up for 3.0000 s
Benchmarking StaticOrDynamic::replace/20: Collecting 100 samples in estimated 5.0117 s (1.3M iterations)
Benchmarking StaticOrDynamic::replace/20: Analyzing
StaticOrDynamic::replace/20
                        time:   [3.8295 µs 3.8363 µs 3.8442 µs]
                        change: [−5.7039% −4.6599% −3.6638%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 10 outliers among 100 measurements (10.00%)
  3 (3.00%) high mild
  7 (7.00%) high severe

Benchmarking PathAndQueryWithSkipped::from_config/simple
Benchmarking PathAndQueryWithSkipped::from_config/simple: Warming up for 3.0000 s
Benchmarking PathAndQueryWithSkipped::from_config/simple: Collecting 100 samples in estimated 5.0004 s (50M iterations)
Benchmarking PathAndQueryWithSkipped::from_config/simple: Analyzing
PathAndQueryWithSkipped::from_config/simple
                        time:   [100.03 ns 100.22 ns 100.42 ns]
                        change: [+1.7769% +2.4954% +3.1891%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) high mild
  1 (1.00%) high severe
Benchmarking PathAndQueryWithSkipped::from_config/3_params
Benchmarking PathAndQueryWithSkipped::from_config/3_params: Warming up for 3.0000 s
Benchmarking PathAndQueryWithSkipped::from_config/3_params: Collecting 100 samples in estimated 5.0001 s (6.8M iterations)
Benchmarking PathAndQueryWithSkipped::from_config/3_params: Analyzing
PathAndQueryWithSkipped::from_config/3_params
                        time:   [750.43 ns 757.85 ns 767.19 ns]
                        change: [−10.366% −9.6443% −8.9205%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  3 (3.00%) low severe
  4 (4.00%) low mild
  2 (2.00%) high severe
Benchmarking PathAndQueryWithSkipped::from_config/10_params
Benchmarking PathAndQueryWithSkipped::from_config/10_params: Warming up for 3.0000 s
Benchmarking PathAndQueryWithSkipped::from_config/10_params: Collecting 100 samples in estimated 5.0011 s (2.1M iterations)
Benchmarking PathAndQueryWithSkipped::from_config/10_params: Analyzing
PathAndQueryWithSkipped::from_config/10_params
                        time:   [2.3765 µs 2.3958 µs 2.4159 µs]
                        change: [−10.604% −9.2655% −7.7836%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  2 (2.00%) low mild
  2 (2.00%) high mild
  3 (3.00%) high severe
Benchmarking PathAndQueryWithSkipped::from_config/10_params_5_marketing
Benchmarking PathAndQueryWithSkipped::from_config/10_params_5_marketing: Warming up for 3.0000 s
Benchmarking PathAndQueryWithSkipped::from_config/10_params_5_marketing: Collecting 100 samples in estimated 5.0057 s (1.1M iterations)
Benchmarking PathAndQueryWithSkipped::from_config/10_params_5_marketing: Analyzing
PathAndQueryWithSkipped::from_config/10_params_5_marketing
                        time:   [4.5749 µs 4.6328 µs 4.7089 µs]
                        change: [−11.811% −11.224% −10.549%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  1 (1.00%) low severe
  1 (1.00%) low mild
  2 (2.00%) high mild
  1 (1.00%) high severe
Benchmarking PathAndQueryWithSkipped::from_config/special_chars
Benchmarking PathAndQueryWithSkipped::from_config/special_chars: Warming up for 3.0000 s
Benchmarking PathAndQueryWithSkipped::from_config/special_chars: Collecting 100 samples in estimated 5.0048 s (3.3M iterations)
Benchmarking PathAndQueryWithSkipped::from_config/special_chars: Analyzing
PathAndQueryWithSkipped::from_config/special_chars
                        time:   [1.4755 µs 1.4865 µs 1.4979 µs]
                        change: [−10.897% −10.039% −9.1285%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 14 outliers among 100 measurements (14.00%)
  4 (4.00%) low severe
  2 (2.00%) low mild
  5 (5.00%) high mild
  3 (3.00%) high severe