refactor: relax generic bounds on CAS/index APIs

[0xdeafbeef]

No behavioural changes

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.66667% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.30%. Comparing base (17953d6) to head (dad4e00).

Files with missing lines	Patch %	Lines
src/lib.rs	60.00%	6 Missing ⚠️
src/index/mod.rs	80.76%	5 Missing ⚠️
src/index/state.rs	83.33%	4 Missing ⚠️
src/transaction.rs	60.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master       #9   +/-   ##
=======================================
  Coverage   82.30%   82.30%           
=======================================
  Files          16       16           
  Lines        2515     2515           
=======================================
  Hits         2070     2070           
  Misses        445      445           

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