Replace uses of imperative SHA2 with spec-adherent SHA2

[marsella]

Several spots throughout the code use the imperative version of SHA2, instead of the more obviously-spec-adherent version added in #112. We should try to replace these with the spec version.

Optional: maybe block on #98 in case some of these uses are actually generic over the hash?

• Identify all uses and determine which need the imperative version specifically
• Decide whether to remove the imperative version. If not, add a README explaining when to use each version. If so, delete it.
• Add notes to SHA-2 spec that it'll be eventually superseded by SHA-3