Squashed 'deps/rust-gmp/' changes from a0544cc..5e7542a

5e7542a Add modulus function

git-subtree-dir: deps/rust-gmp
git-subtree-split: 5e7542a0b62c00a1eea9cf7d58699fb0ba5e1e52
Merge commit 'e546436be4fa225998c2ef41f5f223e1d90b4ead'
Don't build deps constantly

Explicitly build them when they change.
Partial BigInt implementation, modulus, and from_str_radix