Fix semantics of JSONUnkeyedDecodingContainer

[gwynne]

The following behaviors are now handled correctly:

• It is no longer possible for self.count or self.isAtEnd to accidentally get out of sync with reality, they are now computed.
• Bounds are now correctly checked and a more useful DecodingError thrown (may have performance implications; removing this check is not strictly against protocol if so).
• Decoding no longer incorrectly increments the current index if the decode fails for any reason.
• decodeNil() no longer incorrectly increments the current index if the result is not nil.

[gwynne]

If anything I've done affects performance significantly please let me know so I can undo it.

[codecov[bot]]

Codecov Report

Merging #62 into main will increase coverage by 0.32%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   85.24%   85.57%   +0.32%     
==========================================
  Files          13       13              
  Lines        1227     1234       +7     
==========================================
+ Hits         1046     1056      +10     
+ Misses        181      178       -3     

Impacted Files	Coverage Δ
...ftJSON/Decoding/JSONUnkeyedDecodingContainer.swift	98.23% <100.00%> (+2.94%)	:arrow_up:

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[fabianfett]

@gwynne Thanks so much, from what I’ve skimmed this looks awesome. I’m on vacation right now, but I’ll have a thorough look on Sunday.

[fabianfett]

Thanks @gwynne and @calebkleveter!