Refactor XMLCoderElement.flatten, add tests

[MaxDesiatov]

No description provided.

[codecov]

Codecov Report

Merging #88 into master will decrease coverage by 0.68%.
The diff coverage is 77.13%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   78.04%   77.36%   -0.69%     
==========================================
  Files          37       38       +1     
  Lines        2022     2076      +54     
==========================================
+ Hits         1578     1606      +28     
- Misses        444      470      +26

Impacted Files	Coverage Δ
...MLCoder/Decoder/SingleValueDecodingContainer.swift	70% <ø> (ø)
...MLCoder/Encoder/SingleValueEncodingContainer.swift	94.64% <ø> (ø)
Sources/XMLCoder/Auxiliaries/XMLCoderElement.swift	97.17% <100%> (-0.5%)	⬇️
Sources/XMLCoder/Auxiliaries/Box/KeyedBox.swift	100% <100%> (ø)	⬆️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	77.53% <62.68%> (-6.45%)	⬇️
Sources/XMLCoder/Auxiliaries/KeyedStorage.swift	98.8% <98.8%> (ø)

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