Implement bson2.RawDocument checking

[AlekSi]

Description

Improve tests and fuzzing, add benchmarks.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (e54842a) 75.83% compared to head (0d565b5) 74.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4076      +/-   ##
==========================================
- Coverage   75.83%   74.68%   -1.15%     
==========================================
  Files         343      344       +1     
  Lines       21959    21993      +34     
==========================================
- Hits        16652    16426     -226     
- Misses       4021     4299     +278     
+ Partials     1286     1268      -18     

Files	Coverage Δ
internal/bson2/bson2.go	81.66% <ø> (+8.33%)	⬆️
internal/bson2/raw_document.go	80.74% <97.05%> (+8.17%)	⬆️
internal/bson2/decodemode_string.go	0.00% <0.00%> (ø)
internal/bson2/raw_array.go	36.84% <41.66%> (+7.80%)	⬆️

... and 26 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	66.06% <0.00%> (-1.55%)	⬇️
hana-1	?
integration	66.06% <0.00%> (-1.62%)	⬇️
mongodb-1	4.77% <0.00%> (-0.01%)	⬇️
mysql-1	?
mysql-2	?
mysql-3	?
postgresql-1	49.20% <0.00%> (-0.18%)	⬇️
postgresql-2	46.21% <0.00%> (-0.19%)	⬇️
postgresql-3	47.46% <0.00%> (-0.16%)	⬇️
sqlite-1	48.40% <0.00%> (-0.27%)	⬇️
sqlite-2	45.34% <0.00%> (-0.14%)	⬇️
sqlite-3	46.58% <0.00%> (-0.17%)	⬇️
unit	33.05% <74.50%> (+0.19%)	⬆️

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

[AlekSi]

@henvic @rumyantseva PTAL

[rumyantseva]

Looks good!