Add Hash_Engine

[randombit]

This is a limited interface for cryptographic hashing of many messages in parallel, where all of the inputs are exactly the same length. This restriction simplifies the implementation while still being sufficient to meet the needs of hash based signatures.

This provides the interface but the current implementation is only marginally interesting. Future PRs will extend the implementation (given the interface restrictions, it's easy to implement SIMD parallel hashing, AVX512 in particular giving excellent results) and modify the hash-based signature schemes to make use of the available parallelism.

FYI @TJ-91 re #5256; not required to use this in the initial PR but might be of interest

[coveralls]

coverage: 89.492% (-2.4%) from 91.929%
when pulling d6e96a3 on jack/hash-engine
into 507bbbf on master.