Moved _XML…EncodingContainer into their own files, matching decoder

[regexident]

The XMLEncoder.swift is far too large as it is right now.

I thus moved _XMLUnkeyedEncodingContainer and _XMLKeyedEncodingContainer into their own respective files.

Note: The Decoder directory already does the same.

[codecov]

Codecov Report

Merging #4 into master will increase coverage by 4.1%.
The diff coverage is 43.78%.

@@            Coverage Diff            @@
##           master       #4     +/-   ##
=========================================
+ Coverage   34.57%   38.68%   +4.1%     
=========================================
  Files          12       14      +2     
  Lines        1585     1440    -145     
=========================================
+ Hits          548      557      +9     
+ Misses       1037      883    -154

Impacted Files	Coverage Δ
Sources/XMLCoder/XMLKey.swift	25% <ø> (ø)	⬆️
Sources/XMLCoder/ISO8601DateFormatter.swift	0% <ø> (ø)	⬆️
...rces/XMLCoder/Encoder/EncodingErrorExtension.swift	0% <ø> (ø)	⬆️
...XMLCoder/Encoder/XMLUnkeyedEncodingContainer.swift	23.25% <23.25%> (ø)
...s/XMLCoder/Encoder/XMLKeyedEncodingContainer.swift	50.4% <50.4%> (ø)
Sources/XMLCoder/Encoder/XMLEncoder.swift	43.43% <66.66%> (+13.55%)	⬆️

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[regexident]

🤔 this didn't change any logic. Just moving stuff around and changing some stuff from fileprivate to internal.

[MaxDesiatov]

LGTM, again hoping that merge of #1 will make codecov checks pass and then this one's good to merge.