sha: make compress consume blocks

[baloo]

To align with the semantics of block_buffer::BlockBuffer::digest_blocks signature which works with &[Block] and not &[[u8; N]].

[newpavlov]

We intentionally use byte arrays in compression functions for two reasons:

• To prepare ground for future migration to const generics (plus, it's useful for hybrid-array as well).
• To not deal with the somewhat annoying Block imports everywhere.

The compression functions are usually fixed-sized, so I don't think it's worth to expose them to types introduced for dealing with genericity.

[baloo]

• To prepare ground for future migration to const generics (plus, it's useful for hybrid-array as well).

Just to be clear, this was post hybrid-array migration. Block is an alias for hybrid_array::Array already.