Fix decoding of arrays with optional elements

[MaxDesiatov]

Also move protocols for tricks with metatypes to new Metatypes file.

Fix formatting of long lines in related files.

[codecov]

Codecov Report

Merging #48 into master will increase coverage by 0.23%.
The diff coverage is 38.09%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   56.01%   56.24%   +0.23%     
==========================================
  Files          29       30       +1     
  Lines        1414     1433      +19     
==========================================
+ Hits          792      806      +14     
- Misses        622      627       +5

Impacted Files	Coverage Δ
Sources/XMLCoder/Decoder/XMLDecoder.swift	38.64% <ø> (ø)	⬆️
...XMLCoder/Decoder/XMLUnkeyedDecodingContainer.swift	9.16% <0%> (-1.01%)	⬇️
Sources/XMLCoder/Auxiliaries/Metatypes.swift	100% <100%> (ø)
Sources/XMLCoder/Auxiliaries/XMLStackParser.swift	91.78% <100%> (ø)	⬆️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	45.39% <35.29%> (+2.53%)	⬆️
Sources/XMLCoder/Auxiliaries/XMLElement.swift	94.4% <83.33%> (+1.23%)	⬆️

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