[WIP] Minimum modulus size checks

[tarcieri]

Changes the existing checked APIs to respect a minimum modulus size in addition to a maximum one.

Note: several tests fail because of this, so we'll need to go through them and convert to an unchecked API where appropriate (or decide if the test is bogus to begin with).

Closes #445

[tarcieri]

Down to 35 test failures 😅

[pinkforest]

Either superceded by #562 or there is a catch-up merge into this branch in #561

[tarcieri]

I hope this meets @dignifiedquire's requirements. There are escape hatches for both instantiating keys and keygen, so no functionality is actually removed.