WIP copy optimisation

[jodavies]

Here is an optimisation experiment, replacing all NCOPY/WCOPY macros with memmove (we can't be sure that memory regions never overlap in use of the macro for memcpy). The replacement alone is a negligible performance improvement (tentatively 1%?) but it is hard to detect within the usual run-to-run variation.

The followup commits improve some existing copies within the code by using the macros instead, and moving some conditionals outside of the copies. I identified the expensive copies with a profiler running the Forcer benchmark.

On my system (Ryzen 7900X, Ubuntu 24.04, GCC 13.3.0, tform -w12,), the results for the usual benchmarks are as follows:

Benchmark	Speedup w.r.t. v5.0.0
chromatic	1.05 ± 0.01
color	1.01 ± 0.01
fmft	1.03 ± 0.01
forcer	1.07 ± 0.00
forcer-exp	1.08 ± 0.01
mass-fact	1.00 ± 0.05
mbox1l	1.01 ± 0.02
minceex	1.07 ± 0.02
mincer	1.00 ± 0.05
sort-disk	0.98 ± 0.02
sort-large	0.99 ± 0.01
sort-small	1.01 ± 0.01
trace	1.02 ± 0.01