Deduplicate the code

[chreekat]

It would be nice to share as much code as possible between the two libraries.

I've got a branch that takes one approach to this. It builds off my pending PR, so I don't want to open a second PR for it (too much noise in the diff). You can see the changes here, though:

https://github.com/chreekat/pwstore/compare/sync-versions...chreekat:deduplicate-generic-code?expand=1

The strategy used here is to make pwstore-fast depend on pwstore-purehaskell. It just hides the high-level API, re-implements it in terms of its own pbkfd1/2, and re-exports the rest.

The thing is, I'm not sure this is a great solution, since there's still duplication in the export list and in the Haddocks for the high-level API. Any thoughts?