Prefer core::num::NonZero over crypto_bigint::NonZero where possible

[dvdplm]

Tries to address #56 to the extent possible.

TL;DR is that we must use crypto_bigint::NonZero for all non-primitive types.

This PR just makes it explicit which variant we're using by renaming the type to CTNonZero and prefers core's NonZero wherever possible.

Given that ZeroablePrimitive is not on the path to stabilization any time soon, I suggest this closes #56.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.37%. Comparing base (7b88e3e) to head (03309d2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #63   +/-   ##
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  Coverage   99.37%   99.37%           
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  Files           9        9           
  Lines        1280     1280           
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  Hits         1272     1272           
  Misses          8        8           

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