Enforce valid modulus for Residue and associated macros

[AaronFeickert]

This PR follows in the footsteps of #240 and enforces a valid (odd) modulus for compile-time constructions.

It updates impl_modulus! to panic on an invalid modulus.

For Residue, it adds a panic to new and deprecates it, and adds a new_checked constructor that returns a CtOption based on the modulus validity. This is added since it's possible to define a modulus without impl_modulus! and use Residue directly.

It also updates const_residue! to use Residue::new_checked and unwrap, so it will also panic on an invalid modulus.

Closes #232.

[AaronFeickert]

@fjarri: Would appreciate a review if you're available.

[AaronFeickert]

@fjarri: I'll wait to address your review comments until we decide on a common deprecation strategy, as suggested by @tarcieri in #240.

[tarcieri]

Ok, I guess I'll go ahead and merge this, although I'd really, really like to find a way to get rid of new_checked

[AaronFeickert]

Ok, I guess I'll go ahead and merge this, although I'd really, really like to find a way to get rid of new_checked

Agreed. Being able to rely solely on compile-time checks would be much more elegant and in line with the intent of the design, for sure.