Improve data handling for strings and blobs, plus general cleanup

[gwynne]

Several improvements have been made to SQLiteNIO's functionality:

• Less copying (rarely, a lot less copying) around of blobs happens.
• When decoding Strings via the SQLiteDataConvertible path (this primarily affects FluentSQLiteDriver and SQLiteKit), integer and real values are now converted to strings rather than being rejected.
• When decoding Floats and Doubles via the SQLiteDataConvertible path (this primarily affects FluentSQLiteDriver and SQLiteKit), integer values are now converted to the appropriate type rather than being rejected.
• The minimum supported Swift version is now 5.8.
• All EventLoopFuture-based APIs now have explicit async counterparts.
• SQLite statement handles are now finalized correctly after encountering error conditions, which should reduce general memory usage a little.
• SQLite connection handles are no longer leaked when certain errors occur during SQLiteConnection.open().
• SQLiteConnection.open() now throws more useful, less generic errors.
• Strings and binary blobs larger than 4GB are now supported.
• A few places where the code would previously crash on unexpectedly nil values now handle the failures gracefully.
• Further Sendable correctness has been implemented.
• Async APIs avoid excess thread hopping between event loops and tasks whenever possible.
• We now omit several SQLite features that aren't usable through SQLiteNIO anyway.
• Custom user functions are now SQLITE_DIRECTONLY by default; this can be changed by passing indirect: true to the SQLiteCustomFunction initializer if needed.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 55.09138% with 172 lines in your changes are missing coverage. Please review.

Project coverage is 65.60%. Comparing base (2102da7) to head (575470b). Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
+ Coverage   65.03%   65.60%   +0.57%     
==========================================
  Files           8        9       +1     
  Lines         675      756      +81     
==========================================
+ Hits          439      496      +57     
- Misses        236      260      +24     

Files	Coverage Δ
Sources/SQLiteNIO/SQLiteDataType.swift	0.00% <ø> (ø)
Sources/SQLiteNIO/SQLiteRow.swift	64.28% <100.00%> (ø)
Sources/SQLiteNIO/SQLiteData.swift	75.00% <50.00%> (+1.50%)	:arrow_up:
Sources/SQLiteNIO/SQLiteStatement.swift	95.06% <93.61%> (+5.58%)	:arrow_up:
Sources/SQLiteNIO/SQLiteCustomFunction.swift	82.48% <70.00%> (-1.20%)	:arrow_down:
Sources/SQLiteNIO/SQLiteDataConvertible.swift	58.62% <30.43%> (-2.30%)	:arrow_down:
Sources/SQLiteNIO/SQLiteConnection.swift	68.51% <63.15%> (+0.24%)	:arrow_up:
Sources/SQLiteNIO/SQLiteDatabase.swift	30.00% <30.00%> (ø)
Sources/SQLiteNIO/SQLiteError.swift	48.98% <49.73%> (+22.17%)	:arrow_up:

[gwynne]

The dependents test will not pass without the fix to FluentSQLiteDriver to account for the extended result codes in the DatabaseError conformance, but that is not a blocker for this PR.