Split SyntaxError::UnclosedPIOrXmlDecl into separate enum variants

[SuchAFuriousDeath]

Implements #916

The SyntaxError enum is public, so this is a breaking change.
The Parser trait is also public, so the method signature change is also a breaking change.

[SuchAFuriousDeath]

I will do my best to address reviews as fast as I can. Once this is merged, I would appreciate a new release very much :)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.10%. Comparing base (d1acdb5) to head (3388f3c).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
src/errors.rs	0.00%	2 Missing ⚠️
src/reader/state.rs	60.00%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #924      +/-   ##
==========================================
+ Coverage   58.08%   58.10%   +0.01%     
==========================================
  Files          42       42              
  Lines       15513    15522       +9     
==========================================
+ Hits         9011     9019       +8     
- Misses       6502     6503       +1     

Flag	Coverage Δ
unittests	58.10% <77.77%> (+0.01%)	⬆️

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[Mingun]

Probably, that the right way to do that. I haven't looked in detail yet, but I already see a problem with the potential handling of currently unsupported encodings. I would like to see at least #[ignore]d tests added for these cases and FIXMEs in problematic lines.

Also, such change needs changelog entry. I prefer if you rebase your branch over current master instead of merging (that is needed to prevent merge conflict in Changelog.md).