Start rewrite serde serialization tests

[Mingun]

Our serde serializer a far from ideal, but rewriting it requires a very long commit history, as I think (I already have 20 commits and I suspect that I'll double that count...). Because I'm not a big fan of long PRs, I propose that changes, that rewrites tests without changing behavior. Next tests that I plan to add will test new corrected behavior, that is why this PR could look slightly unfinished.

Added a commit that ensures, that key name is always produces valid XML name for an element

[codecov-commenter]

Codecov Report

Merging #468 (a5e03b5) into master (6bedf6c) will decrease coverage by 1.65%.
The diff coverage is 88.50%.

@@            Coverage Diff             @@
##           master     #468      +/-   ##
==========================================
- Coverage   52.70%   51.05%   -1.66%     
==========================================
  Files          29       30       +1     
  Lines       13535    13143     -392     
==========================================
- Hits         7134     6710     -424     
- Misses       6401     6433      +32     

Flag	Coverage Δ
unittests	51.05% <88.50%> (-1.66%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/de/escape.rs	21.05% <0.00%> (ø)
src/de/map.rs	72.07% <0.00%> (-0.66%)	⬇️
src/de/var.rs	85.10% <0.00%> (ø)
src/errors.rs	2.06% <0.00%> (-0.02%)	⬇️
src/utils.rs	64.84% <50.00%> (-2.12%)	⬇️
src/se/mod.rs	80.44% <90.69%> (-13.37%)	⬇️
src/se/key.rs	92.23% <92.23%> (ø)
src/de/mod.rs	82.02% <100.00%> (-0.43%)	⬇️
src/de/simple_type.rs	90.63% <100.00%> (ø)
src/se/var.rs	93.95% <100.00%> (+1.39%)	⬆️
... and 5 more

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[dralley]

I'd like to try to not be rewriting the serde code in parallel - if there are relevant pieces of #441 which you would be willing to merge separately, let's try to do that.

[Mingun]

For now I see that we are practically do not intersect -- it seems that serializer implementation does not depend much on the encoding, because it always writes UTF-8 strings. I rewriting serializer completely, so it will use std::fmt::Write (instead of std::io::Write) and not deal with encodings at all

[dralley]

Ensure that map keys are restricted to only types that can be serialized as primitives