Update PrimeParams to require a multiplicative generator value

[andrewwhitehead]

This also adjusts the const_prime_monty_params! macro to accept the new generator parameter. The t constant is added to PrimeParams for convenience.

[codecov]

Codecov Report

❌ Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.94%. Comparing base (3c38f48) to head (e9e77e5).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/modular/prime_params.rs	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
- Coverage   87.96%   87.94%   -0.03%     
==========================================
  Files         184      184              
  Lines       21172    21165       -7     
==========================================
- Hits        18624    18613      -11     
- Misses       2548     2552       +4     

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[tarcieri]

This is a breaking change, which we technically can't make anymore, though it does seem like a very useful one for elliptic-curve so we can probably get away with one final breaking change and yanking the v0.7.0 release.

(I was thinking about waiting to see if you wanted to make a change like this before releasing v0.7.0, but I guess I jumped the gun)

[fjarri]

Or just release a 0.8? :)