Use native fromBase64 if possible, instead of atob.

[hildjj]

Ensure fallback implementations of Uint8Array methods are always tested.

Don't check for Uint8Array methods on each call.

Follow-on to #108.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b25d827) to head (26a82f6).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #111   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines         5183      5234   +51     
  Branches       750       752    +2     
=========================================
+ Hits          5183      5234   +51     

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