Mpq: Implement From<i32>, From<u32>
Mpq: Implement From<i32>, From<u32>

This allows one to write Mpq::from(17) instead of the cumbersome
Mpq::from(17i64).

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

fizyk20/rust-gmp