Re-sort the TODO
Use blake2b unconditionally.

I had previously thought "hey, maybe I'll use blake2s on 32-bit hosts",
but the performance of the digest doesn't matter, since we only use
it during entropy compression.

Also rename WORDBITS to BLAKE2_WORDBITS so we don't confuse it for
something else.
Remove workarounds for blake2s

Previously, I had some workaround code to keep us happy when the hash
output is less than the length of the needed key material.  But now
that we're blake2b only, I can rip that out.
Support (separately) heap-allocated RNG structs

This is a prelude to using mmap to allocate them off in cloudcookoo
land.
Fix a cut-and-paste comment
Use mmap for rng allocation