Make impl_modulus! usable from external crates

[haslersn]

Previously it was impossible to use impl_modulus! in crates that use crypto-bigint, because certain functions were only available within the crypto-bigint crate.

[mikelodder7]

Makes sense. Nothing else better we could do than define macros to create constants?

[tarcieri]

Not for now. When const generics improve we can get rid of the macros.

[fjarri]

I haven't been wild about their name. They're really "const_" as opposed to "ct_", and the wrapper functions which return subtle-based types like CtOption and Choice are also constant-time, which makes the ct_ prefix confusing in the public API.

I would vote for making the const fns have const_ prefix. They won't intersect with the subtle-based functions since those can't be used in the const context anyway. We don't need a special ct_ prefix for constant-timeness since the library already postulates that it's constant-time by default, and anything not constant-time is specifically marked as _vartime. The only problem are the const fn analogues of subtle methods which have ct_ prefixes for consistency reasons, but I think they're all currently crate-private.

Edit: on a second thought, in many cases we don't even need the const_ prefix since there are no non-const counterparts (e.g. bits()).

[tarcieri]

@haslersn can you add a test to the tests/ directory to ensure that impl_modulus! works via the crate's public API?

Otherwise I'd say this is fine and we can circle back on the ct_ prefix separately.

[haslersn]

Should now be all done including the const_ prefix:

• Added test
• Added $crate qualification in the impl_modulus! implementation so that the macro's user doesn't need additional imports
• Renamed ct_rem{_wide} to const_rem{_wide}

[tarcieri]

@haslersn hmm, I don't see the test?

[haslersn]

I don't see the test

Oops, git add -p doesn't cover new files.

[tarcieri]

@haslersn think you still need to push the changes

[haslersn]

No, they were pushed just before my previous comment

[tarcieri]

Oh sorry, didn’t refresh