Experiment with SQLCipher

[rnewman]

SQLCipher is a fork of SQLite that transparently encrypts the database file.

In theory, it's a drop-in replacement with only one change: specifying a key when opening the database.

With no key, SQLCipher should behave exactly the same as SQLite.

We should see if we can accommodate both in our API, and see what performance is like.

[rnewman]

I think this is an important differentiator, and I'd like to see it explored sooner rather than later.

This is also a good first bug for a certain kind of contributor: after #171 and #148 we're in a position to use and exercise rusqlite, and it would be interesting to look at https://github.com/jgallagher/rusqlite/issues/219 and figure out what the build steps would be.