fast-modulo

This crate needs nightly. (for feature(asm))

benchmark

OS: Ubuntu 18.04, CPU: AMD Ryzen 7 2700X, rustc 1.50.0-nightly (bb178237c 2020-12-25) ``` reference::mulmod_u64 time: [23.264 ns 23.277 ns 23.292 ns]
Found 3 outliers among 100 measurements (3.00%) 1 (1.00%) low mild 2 (2.00%) high mild

mulmod_u64 time: [10.253 ns 10.256 ns 10.259 ns]
Found 4 outliers among 100 measurements (4.00%) 1 (1.00%) low mild 2 (2.00%) high mild 1 (1.00%) high severe

reference::mod_u128u64 time: [37.003 ns 37.012 ns 37.021 ns]
Found 3 outliers among 100 measurements (3.00%) 2 (2.00%) high mild 1 (1.00%) high severe

mod_u128u64 time: [13.852 ns 13.856 ns 13.861 ns]

reference::powmod_u64 time: [2.2661 us 2.2671 us 2.2680 us]
Found 4 outliers among 100 measurements (4.00%) 2 (2.00%) high mild 2 (2.00%) high severe

powmod_u64 time: [916.55 ns 916.74 ns 916.93 ns]
Found 4 outliers among 100 measurements (4.00%) 3 (3.00%) high mild 1 (1.00%) high severe ```