Add rxml to comparison and tweak benchmarks compile options

[Mingun]

rxml is a crate under the hood of minidom, which in the past used quick-xml as XML parser. Therefore it will be useful to compare it performance.

Enable LTO for maximum performance.
Enable debug information because that makes cargo flamegraph output more useful.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.55%. Comparing base (d1acdb5) to head (e15e4da).
⚠️ Report is 19 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #931      +/-   ##
==========================================
- Coverage   58.08%   54.55%   -3.54%     
==========================================
  Files          42       44       +2     
  Lines       15513    16909    +1396     
==========================================
+ Hits         9011     9225     +214     
- Misses       6502     7684    +1182     

Flag	Coverage Δ
unittests	54.55% <ø> (-3.54%)	⬇️

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