Remove some of the swiftlint generated warnings

[MartinP7r]

This PR tends to some of the warnings generated by swiftlint.

Please feel free to adjust if the changes don't work for you.

The FIXME on the test values in StringTests.swift:19 seems to actually be fixed somehow because the tests are passing without any adjustment.

[codecov]

Codecov Report

Merging #229 (c8a0f83) into main (887de88) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   73.94%   74.00%   +0.06%     
==========================================
  Files          46       46              
  Lines        2437     2443       +6     
==========================================
+ Hits         1802     1808       +6     
  Misses        635      635              

Impacted Files	Coverage Δ
Sources/XMLCoder/Auxiliaries/XMLStackParser.swift	94.05% <ø> (+0.12%)	⬆️
...rces/XMLCoder/Decoder/DecodingErrorExtension.swift	68.00% <ø> (ø)
Sources/XMLCoder/Decoder/XMLDecoder.swift	78.72% <ø> (ø)
Sources/XMLCoder/Auxiliaries/XMLCoderElement.swift	96.55% <0.00%> (+0.06%)	⬆️

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[MaxDesiatov]

Thanks 👍