Constructors for scalars from u128

[nickray]

These are helpful when using the scalars in Shamir secret sharing with random (128 bit) participants.

I didn't touch the field.rs files, as then automatic method selection in the multiplicative_generator implementations fails - so there is potentially a danger of backwards incompat in p256+p384 which don't have From<u32>.

Personally, I'm only interested in From<u128> for k256.

[tarcieri]

It seems this breaks inference for integer conversions where the type isn't annotated, so you'll need to annotate the types where they weren't annotated before

[nickray]

Right, I missed the 32-bit build test.

[tarcieri]

Thanks!