Fix handling of array bindings containing optional values

[gwynne]

These changes are now available in 2.16.0

• Now requires Swift 6.1
• We now use SQLKit's SomeCodingKey instead of redefining it.
• Encoding of nil binds to NULL is now on the fastest possible path instead of the slowest.
• Values that conform to Postgres[Throwing]DynamicTypeEncodable but not Postgres[NonThrowing]Encodable are now handled on the fast path.
• nil values in bound arrays having optional element types (which they should, as all arrays in Postgres have NULLable values) are now handled correctly instead of causing crashes.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.37%. Comparing base (e4f9052) to head (b772d81).

Files with missing lines	Patch %	Lines
Sources/PostgresKit/PostgresDataTranslation.swift	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #279      +/-   ##
==========================================
+ Coverage   42.84%   43.37%   +0.53%     
==========================================
  Files          13       13              
  Lines         852      853       +1     
==========================================
+ Hits          365      370       +5     
+ Misses        487      483       -4     

Files with missing lines	Coverage Δ
Sources/PostgresKit/PostgresDataTranslation.swift	70.37% <80.00%> (+1.59%)	⬆️

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[0xTim]

We can point to the API docs if that's better