Add index<I>() to Array-like types

[ktbarrett]

This mirrors slice<R>(). The main value is that static indexing into statically ranged objects is bounds-checked at compile-time.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.05%. Comparing base (27d6b45) to head (645c86c).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #289   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files          23       23           
  Lines        1309     1309           
  Branches      316      316           
=======================================
  Hits         1205     1205           
  Misses         43       43           
  Partials       61       61           

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