Honor .sortedKeys (redux)

[qmoya]

This PR supersedes #11, which had some unwanted commits added to it. It addresses the formatting issues mentioned by @MaxDesiatov.

[codecov]

Codecov Report

Merging #14 into master will increase coverage by 1.41%.
The diff coverage is 23.3%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   24.58%   25.99%   +1.41%     
==========================================
  Files          12       12              
  Lines        1554     1585      +31     
==========================================
+ Hits          382      412      +30     
- Misses       1172     1173       +1

Impacted Files	Coverage Δ
Sources/XMLCoder/ISO8601DateFormatter.swift	0% <ø> (ø)	⬆️
...rces/XMLCoder/Decoder/DecodingErrorExtension.swift	0% <ø> (ø)	⬆️
...urces/XMLCoder/Encoder/XMLReferencingEncoder.swift	0% <0%> (ø)	⬆️
...rces/XMLCoder/Encoder/EncodingErrorExtension.swift	0% <0%> (ø)	⬆️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	11.88% <10.92%> (ø)	⬆️
Sources/XMLCoder/Decoder/XMLDecoder.swift	22.61% <19.04%> (ø)	⬆️
Sources/XMLCoder/Encoder/XMLEncoder.swift	22.93% <20.85%> (+0.04%)	⬆️
Sources/XMLCoder/XMLKey.swift	25% <50%> (ø)	⬆️
...XMLCoder/Decoder/XMLUnkeyedDecodingContainer.swift	7.56% <6.74%> (ø)	⬆️
Sources/XMLCoder/Encoder/XMLEncodingStorage.swift	87.5% <83.33%> (ø)	⬆️
... and 2 more

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[qmoya]

Hi @MaxDesiatov: codecov/patch failed because the PR doesn't have tests for all the files that have been reformatted by SwiftFormat. 😄 But I believe it's ready to merge.

Once you do that, I'll suggest another PR that adds character data support for encoding and decoding.

Thanks!

[MaxDesiatov]

LGTM, thanks!