Struct sendable

[swhitty]

Convert the top level types KeyValueEncoder and KeyValueDecoder from class to struct so they can easily become Sendable without adding a lock.

While this is a API change breaking change the fixit is easy convert let decoder to var decoder where required.

Only code that adjusts the decoding strategies will be impacted.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.88%. Comparing base (45154f9) to head (2f9f5bd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #11   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files           3        3           
  Lines         891      891           
=======================================
  Hits          890      890           
  Misses          1        1           

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