Release SFMT v0.4.0, SIMD-oriented Fast Mersenne Twister

I’ve release the v0.4.0 of sfmt crate, a Rust implementation of SIMD-oriented Fast Mersenne Twister (SFMT) using std::arch. This is by hand re-impementation of original C code in pure Rust. While MT is not a CSPRNG, MT has a long period (2^19973-1) and is suitable for Monte-Carlo simulations.

v0.4.0 drops packed_simd in order to work on stable channel, and switches to Rust2018 (thus works on beta channel). This adopts rand 0.5 interface.