EOS pluggable Digest: fix or remove

[chrisdickinson]

SO! I have a comment here:

// TODO: rip out all the "generic digest" stuff. It's kudzu.

Ripping out the generic digest in favor of concretely requiring Sha256 would be great. ALTERNATIVELY, if you can figure out how to get at the size of the output hash and allow us to vary return types (e.g., the [u8; 32] here) it may be worth retaining.