Optionally use wide crate for DCT, which adds support for ARM64, WASM and SSE2 on stable

[mcroomp]

The result is a 50% speedup for non-AVX SSE2 (default for rust x86_64). If compiling for AVX2, the code is about 20% slower than the hand-rolled AVX2 code.

The wide crate already has safe abstractions for most of the common CPU architectures, so this means we support for WASM and ARM64 Neon CPUs.

Wide works on stable.