Make PSQLDecodingContext generic

[fabianfett]

Motivation

Since we don't attach the JSONDecoder to the connection anymore and supply during decoding, we can now make the decoding generic over the supplied JSONDecoder.

Changes

• Make PSQLDecodingContext generic over the JSONDecoder
• Rename PSQLDecodingContext to PostgresDecodingContext

Result

• We don't use an existential in the very hot code paths.

[codecov-commenter]

Codecov Report

Merging #217 (08218a3) into main (3ee1245) will increase coverage by 4.00%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
+ Coverage   43.04%   47.04%   +4.00%     
==========================================
  Files         119      119              
  Lines        8231     8226       -5     
==========================================
+ Hits         3543     3870     +327     
+ Misses       4688     4356     -332     

Flag	Coverage Δ
unittests	47.04% <81.81%> (+4.00%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Sources/PostgresNIO/Postgres+PSQLCompat.swift	0.00% <0.00%> (ø)
Sources/PostgresNIO/New/Data/Int+PSQLCodable.swift	28.10% <40.00%> (+5.88%)	⬆️
...urces/PostgresNIO/New/Data/Array+PSQLCodable.swift	94.68% <100.00%> (+1.06%)	⬆️
...ources/PostgresNIO/New/Data/Bool+PSQLCodable.swift	95.34% <100.00%> (+4.65%)	⬆️
...urces/PostgresNIO/New/Data/Bytes+PSQLCodable.swift	73.52% <100.00%> (ø)
...ources/PostgresNIO/New/Data/Date+PSQLCodable.swift	89.28% <100.00%> (ø)
...ces/PostgresNIO/New/Data/Decimal+PSQLCodable.swift	74.19% <100.00%> (+6.45%)	⬆️
...urces/PostgresNIO/New/Data/Float+PSQLCodable.swift	76.66% <100.00%> (+3.33%)	⬆️
...ources/PostgresNIO/New/Data/JSON+PSQLCodable.swift	86.95% <100.00%> (ø)
...es/PostgresNIO/New/Data/Optional+PSQLCodable.swift	61.11% <100.00%> (-2.05%)	⬇️
... and 32 more

[gwynne]

Love it. Ship it! ❤️‍🔥