Simplifying and standardizing hashing logic in akd_core

[kevinlewi]

There are a couple of places in the code that we should alter the hashing logic for, mainly for performance. I will add references to this issue in the codebase to keep track of these spots, each with a suggestion in the comments for how it should be changed.

Note that this will introduce breaking changes, and so databases built from previous versions of the code will be incompatible.