Port of ref/hare/hash/hash.ha — the general-purpose hashing-function
interface that crypto/sha256 (and, later, the lib/hash/* checksums)
embed as their first field. Rides the proven io.stream + inline-vtable
shape: a hash is an io-write-only stream plus sum()/reset()/sz()/bsz().
One divergence (documented at-site): Hare's `stream: io::stream` field
becomes an inline `vt: io.vtable` at offset 0. ww's io collapse (#94)
makes the dispatchers take the vtable pointer directly, so the vtable
must be embedded inline for a *state to recover from the dispatch arg —
the base64/memio/io convention.
Hojun-Cho
165dd7388d