Clean up XMLElement, ArrayBox & DictionaryBox

[regexident]

Depends on #26 & #27. Not anymore.

This is a first attempt at making _XMLElement more approachable.

[codecov]

Codecov Report

Merging #28 into master will increase coverage by 0.26%.
The diff coverage is 75.84%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   51.31%   51.57%   +0.26%     
==========================================
  Files          29       29              
  Lines        1374     1400      +26     
==========================================
+ Hits          705      722      +17     
- Misses        669      678       +9

Impacted Files	Coverage Δ
...urces/XMLCoder/Encoder/XMLReferencingEncoder.swift	0% <0%> (ø)	⬆️
...s/XMLCoder/Encoder/XMLKeyedEncodingContainer.swift	42.16% <0%> (ø)	⬆️
Sources/XMLCoder/Auxiliaries/XMLStackParser.swift	86.53% <100%> (+2.6%)	⬆️
Sources/XMLCoder/Decoder/XMLDecoder.swift	39.69% <100%> (ø)	⬆️
Sources/XMLCoder/Encoder/XMLEncoder.swift	48.21% <50%> (ø)	⬆️
Sources/XMLCoder/Box/UnkeyedBox.swift	57.14% <50%> (ø)	⬆️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	38.77% <58.06%> (+3.16%)	⬆️
Sources/XMLCoder/Box/KeyedBox.swift	77.27% <75.6%> (+3.07%)	⬆️
Sources/XMLCoder/Auxiliaries/XMLElement.swift	87.12% <82.05%> (-2.88%)	⬇️
... and 1 more

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[regexident]

"Removed obscure use of static func in XMLElement in favor of simple initializers and instance methods" 🎉🥳

[regexident]

Turned _XMLElement from a class into a struct! 🎉🥳

[regexident]

Should be ready to merge now. 🙂

Try to merge #27 first.
(Looks like I somehow rebased on that one. Unintentionally. Again. 🙄)