Add hardware-accelerated implementation of SHA256

[zah]

Most modern CPUs offer specialized instructions for computing the SHA family of functions. For optimal power usage and maximum throughput in SSZ merkleization, we can benefit from such a hardware-accelerated implementation in Nimcrypto.

Existing code in other languages:
https://github.com/noloader/SHA-Intrinsics
https://github.com/minio/sha256-simd

[mratsim]

Code from a cryptographer (and also co-author in hash-to-curve draft)

https://github.com/armfazh/flo-shani-aesni