The max digest size for the foreseeable future will be 512 bits, and the
max block size is currently 1152 bits (SHA3-224). If people add more
exotic hash algorithms without bumping the constants when required,
tests will fail.
The stream buffer will currently be 576 bytes, which is "fine" to just
stick on the stack, and is a sensible multiple of the more common block
size of 64 bytes.
There is a lot of code duplicated in convenience methods in each hash
implementation, and having a generic hash type makes implementing
higher-level constructs such as HMAC significantly easier down the road.
