Add PostgresCell

[fabianfett]

Motivation

In #219 we want to restructure PostgresRow and introduce PostgresRandomAccessRow. To make it very explicit, it would be cool if PostgresRow was implementing Collection and PostgresRandomAccessRow was implementing RandomAccessCollection. However to do this we need an Element type. This is what PostgresCell shall be.

Changes

• Add PostgresCell
• Remove PSQLData

Result

We have an awesome Element type for PostgresRow.

[codecov-commenter]

Codecov Report

Merging #220 (2c86c18) into main (4937286) will increase coverage by 4.91%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
+ Coverage   43.19%   48.10%   +4.91%     
==========================================
  Files         119      119              
  Lines        8210     8243      +33     
==========================================
+ Hits         3546     3965     +419     
+ Misses       4664     4278     -386     

Flag	Coverage Δ
unittests	48.10% <96.77%> (+4.91%)	⬆️

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

Impacted Files	Coverage Δ
Sources/PostgresNIO/New/PostgresCell.swift	96.77% <96.77%> (ø)
...urces/PostgresNIO/New/Data/Array+PSQLCodable.swift	94.68% <0.00%> (+1.06%)	⬆️
...rces/PostgresNIO/New/Extensions/Logging+PSQL.swift	19.60% <0.00%> (+1.42%)	⬆️
Sources/PostgresNIO/New/PSQLConnection.swift	31.68% <0.00%> (+1.98%)	⬆️
...s/PostgresNIO/Message/PostgresMessageDecoder.swift	95.45% <0.00%> (+2.27%)	⬆️
...es/PostgresNIO/New/PSQLBackendMessageDecoder.swift	93.28% <0.00%> (+2.98%)	⬆️
...urces/PostgresNIO/New/Messages/ErrorResponse.swift	32.30% <0.00%> (+3.07%)	⬆️
...urces/PostgresNIO/New/Data/Float+PSQLCodable.swift	76.66% <0.00%> (+3.33%)	⬆️
Sources/PostgresNIO/New/PSQLBackendMessage.swift	96.63% <0.00%> (+3.36%)	⬆️
...ources/PostgresNIO/Data/PostgresData+Numeric.swift	70.44% <0.00%> (+3.64%)	⬆️
... and 20 more

[gwynne]

Other than the one concern, I don't see anything that needs changing... is there a more specific reason to keep this PR separate other than general desire to cut down on large PRs? (A worthy goal, to be sure!)

[gwynne]

Merge away!