DataRow without allocation; DataRow as Collection; RowDescription top level

[fabianfett]

This is a cherry pick of #188.

Modifications

• DataRow and RowDescription have been moved out of the PSQLBackendMessage namespace. This allows us to mark them as @inlinable or @usableFromInline at a later point, without marking everything in PSQLBackendMessage as @inlinable
• DataRow does not use an internal array for its columns anymore. Instead all read operations are directly done on its ByteBuffer slice.
• DataRow implements the Collection protocol now.

Result

One allocation fewer per queried row.

[codecov-commenter]

Codecov Report

Merging #198 (02aa7fc) into main (9967360) will increase coverage by 4.10%.
The diff coverage is 86.04%.

❗ Current head 02aa7fc differs from pull request most recent head e8bc674. Consider uploading reports for the commit e8bc674 to get more accurate results

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
+ Coverage   35.75%   39.85%   +4.10%     
==========================================
  Files         115      115              
  Lines        7741     7795      +54     
==========================================
+ Hits         2768     3107     +339     
+ Misses       4973     4688     -285     

Flag	Coverage Δ
unittests	39.85% <86.04%> (+4.10%)	⬆️

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

Impacted Files	Coverage Δ
...esNIO/Connection/PostgresConnection+Database.swift	0.00% <0.00%> (ø)
Sources/PostgresNIO/New/PSQLChannelHandler.swift	58.45% <ø> (+10.49%)	⬆️
Sources/PostgresNIO/New/PSQLConnection.swift	31.55% <0.00%> (+1.94%)	⬆️
Sources/PostgresNIO/New/PSQLRow.swift	0.00% <0.00%> (ø)
Sources/PostgresNIO/New/PSQLRowStream.swift	0.00% <0.00%> (ø)
Sources/PostgresNIO/New/PSQLTask.swift	35.13% <ø> (ø)
Sources/PostgresNIO/New/Messages/DataRow.swift	95.94% <95.52%> (-4.06%)	⬇️
...nection State Machine/ConnectionStateMachine.swift	54.82% <100.00%> (+8.88%)	⬆️
...tion State Machine/ExtendedQueryStateMachine.swift	65.26% <100.00%> (+7.78%)	⬆️
...n State Machine/PrepareStatementStateMachine.swift	53.39% <100.00%> (+4.85%)	⬆️
... and 27 more

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[gwynne]

A few minor nits and questions, nothing serious.

[gwynne]

LGTM now 🙂

[VaporBot]

These changes are now available in 1.6.4