Explicitly disallow duplicate keys in data documents

[rumyantseva]

Description

Closes #364.

As agreed with the team, quite a big refactoring was made. Now types.Document's internal representation consists of a slice of fields where each field has a string field and any value.
That approach allows having duplicated keys in documents in principle.

However, Document.ValidateData checks that there are no duplicates.

Minor refactoring: Document.Set's signature doesn't return error anymore, as, in fact, the implementation always returns nil. With that, there is no need to make the code more complicated (with all the checks for errors).

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #1293 (0d5e4bb) into main (38b30b5) will increase coverage by 0.09%.
The diff coverage is 85.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1293      +/-   ##
==========================================
+ Coverage   67.31%   67.40%   +0.09%     
==========================================
  Files         276      276              
  Lines       13024    13021       -3     
==========================================
+ Hits         8767     8777      +10     
+ Misses       3337     3325      -12     
+ Partials      920      919       -1     

Impacted Files	Coverage Δ
internal/handlers/tigris/msg_getparameter.go	0.00% <0.00%> (ø)
internal/util/version/version.go	48.52% <0.00%> (-4.42%)	⬇️
internal/handlers/tigris/msg_findandmodify.go	53.23% <60.00%> (ø)
internal/handlers/tigris/msg_update.go	51.63% <60.00%> (+0.31%)	⬆️
internal/types/document_validation.go	86.48% <84.61%> (-6.62%)	⬇️
internal/types/document.go	88.04% <88.40%> (+0.69%)	⬆️
internal/handlers/common/update.go	87.38% <93.75%> (+3.63%)	⬆️
internal/handlers/common/error.go	80.34% <100.00%> (ø)
internal/handlers/common/projection.go	64.37% <100.00%> (ø)
internal/handlers/common/serverstatus.go	86.95% <100.00%> (ø)
... and 25 more

Flag	Coverage Δ
integration	63.31% <81.59%> (+0.09%)	⬆️
mongodb	14.19% <30.67%> (+0.06%)	⬆️
pg	52.86% <76.68%> (+0.12%)	⬆️
tigris	34.68% <58.89%> (+0.10%)	⬆️
unit	25.60% <53.37%> (+0.09%)	⬆️

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

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[chilagrow]

Thanks for great refactor too 💪

[rumyantseva]

@w84thesun, answering your questions about testing duplicates. Unfortunately, currently, such tests would fail because of #1263. I added a test with skip comment.