Future use of `core::simd` and moving to SIMD-based vectorized implementations

[brycx]

Portable SIMD has landed in nightly Rust core::simd[1]. We should investigate if using SIMD in Orion is a viable option. For example, ChaCha20 can be vectorized[2], which could provide quite substantial performance benefits.

What needs to be figured out:

• What is, if there is any, the timeline for stabilization of core::simd and potentially std::simd (?)
• Will the portable SIMD API require use of unsafe code if utilized within Orion?
• What other primitives could benefit from vectorization? BLAKE2 is a candidate as well IIRC
• If simd would require std for some unknown reason, do we want to consider providing vectorized implementations for safe-api only?
• Will the portable SIMD offered by Rust be portable enough in the sense that we'd be able to remove the current portable non-vectorized implementations?
• Would the portable SIMD be usable on all platforms we test on currently, including the non-std targets? What would the impact on WASM support be here?

[1]: Most of the work seems to happen over here https://github.com/rust-lang/portable-simd [2]: https://eprint.iacr.org/2013/759.pdf