Define native word size as `constant_time::Word`.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 97.23%. Comparing base (731f419) to head (e8944f7). Report is 1 commits behind head on main.

Files	Patch %	Lines
src/polyfill.rs	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2097 +/- ## ========================================== - Coverage 97.25% 97.23% -0.02% ========================================== Files 143 143 Lines 20052 20055 +3 Branches 228 228 ========================================== - Hits 19502 19501 -1 - Misses 524 527 +3 - Partials 26 27 +1 ```

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