Add to_utf8_io_writer helper serializing into std::io::Write

[pronebird]

This PR is related to #499 and adds a helper method to dump XML into std::io::Write restricted to utf-8 output. The name is a bit verbose trying to convey that a. it's utf-8 only b. it's io::Writer and not fmt::Write to avoid confusion

[Mingun]

Could you also add a changelog entry under New Features? Do not forgot to add a link to PR, like other entries. All links placed at the end of corresponding version section (Unreleased in that case)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 60.31%. Comparing base (a9391f3) to head (c0413a3).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
src/se/mod.rs	0.00%	7 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #836      +/-   ##
==========================================
+ Coverage   60.21%   60.31%   +0.09%     
==========================================
  Files          41       41              
  Lines       16021    16002      -19     
==========================================
+ Hits         9647     9651       +4     
+ Misses       6374     6351      -23     

Flag	Coverage Δ
unittests	60.31% <0.00%> (+0.09%)	⬆️

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[pronebird]

Could you also add a changelog entry under New Features? Do not forgot to add a link to PR, like other entries. All links placed at the end of corresponding version section (Unreleased in that case)

Sure thing. Done.