Cleanup PostgresDecodable

[fabianfett]

Motivation

Before we make the PostgresDecodable public, let's revisit it. In Swift prefer initializer over static named methods for type creation.

Changes

• Replace the static func decode with an init
• Cleanup the default implementations. Put the impl. into their correct protocol conformance.

[fabianfett]

@gwynne This is the real meat of this PR. Everything else is cleanup.

[codecov-commenter]

Codecov Report

Merging #241 (8d9f229) into main (d16467d) will increase coverage by 4.04%.
The diff coverage is 77.96%.

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
+ Coverage   39.56%   43.61%   +4.04%     
==========================================
  Files         115      115              
  Lines        9478     9486       +8     
==========================================
+ Hits         3750     4137     +387     
+ Misses       5728     5349     -379     

Flag	Coverage Δ
unittests	43.61% <77.96%> (+4.04%)	⬆️

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

Impacted Files	Coverage Δ
...ces/PostgresNIO/New/Data/Int+PostgresCodable.swift	30.71% <37.14%> (+5.22%)	⬆️
Sources/PostgresNIO/New/PostgresCodable.swift	91.66% <66.66%> (ø)
...s/PostgresNIO/New/Data/Float+PostgresCodable.swift	76.66% <85.71%> (+3.33%)	⬆️
...PostgresNIO/New/Data/Decimal+PostgresCodable.swift	74.19% <91.66%> (+6.45%)	⬆️
...s/PostgresNIO/New/Data/Array+PostgresCodable.swift	94.73% <100.00%> (+1.11%)	⬆️
...es/PostgresNIO/New/Data/Bool+PostgresCodable.swift	95.34% <100.00%> (+4.65%)	⬆️
...s/PostgresNIO/New/Data/Bytes+PostgresCodable.swift	73.52% <100.00%> (ø)
...es/PostgresNIO/New/Data/Date+PostgresCodable.swift	89.28% <100.00%> (ø)
...es/PostgresNIO/New/Data/JSON+PostgresCodable.swift	100.00% <100.00%> (ø)
...IO/New/Data/RawRepresentable+PostgresCodable.swift	82.35% <100.00%> (ø)
... and 32 more

[gwynne]

👍