EOS pluggable Digest: fix or remove

SO! I have a comment [here](https://github.com/entropic-dev/eos-spike/blob/latest/src/envelope.rs#L29):

> `// 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](https://github.com/entropic-dev/eos-spike/blob/latest/src/envelope.rs#L30)) it may be worth retaining.