Remove concrete encode/decode functions

[hodovani]

I removed unnecessery typed encode/decode functions in Keyed and Unkeyed Encoder and Decoder. It seems to me that generic functions work well and we don't need to have these ones.

@regexident could you please comment this solution.

[codecov]

Codecov Report

Merging #64 into master will increase coverage by 2.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage    74.7%   76.93%   +2.22%     
==========================================
  Files          31       31              
  Lines        1550     1396     -154     
==========================================
- Hits         1158     1074      -84     
+ Misses        392      322      -70

Impacted Files	Coverage Δ
...XMLCoder/Encoder/XMLUnkeyedEncodingContainer.swift	31.42% <ø> (+17.67%)	⬆️
...XMLCoder/Decoder/XMLUnkeyedDecodingContainer.swift	15.23% <ø> (-10.3%)	⬇️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	82.5% <ø> (-2.61%)	⬇️
...s/XMLCoder/Encoder/XMLKeyedEncodingContainer.swift	40.42% <ø> (-19.36%)	⬇️

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[MaxDesiatov]

This is a fantastic spot, thanks @hodovani! it never occurred to me that these functions might have a default implementation. It also looks like this on average slightly improves performance 🤩