Squashed 'deps/rust-gmp/' changes from 5e7542a..c4a8ff4

c4a8ff4 Add div_floor and mod_floor

git-subtree-dir: deps/rust-gmp
git-subtree-split: c4a8ff4799ed950eb836050670f704c645c07fc5
Merge commit '8113f8ba9d7d52e69d1513c8c4976041de49a0eb'
Implement num::Integer

Whoops, no tests, will add later