Updates for sendable closures

[johnfairh]

Sendable working properly in beta 3 - not sure where GHA is though.

Think this is looking OK - Linux stuck behind.

Bit concerned there are differences on the CLI vs Xcode - suspect I do have to move to require Swift 6 from the Package.swift, too hard to not use language-mode=6 in Xcode.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (a20885b) to head (8570b8e).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
- Coverage   98.06%   98.03%   -0.04%     
==========================================
  Files          22       22              
  Lines        2068     1527     -541     
==========================================
- Hits         2028     1497     -531     
+ Misses         40       30      -10     

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