Add sqlx support and test

[paulgb]

This is an alternative approach to #7. It adds support for use in an sqlx AsRow derive, by implementing TryFrom<Vec<u8>> and TryFrom<Option<Vec<u8>>>.

One limitation of this approach is that for a nullable column, we can only return a FractionalIndex, not an Option<FractionalIndex>. I wish sqlx would implement something like #[sqlx(try_from_option = "Vec<u8>")] that would handle nullable columns by returning a None, but unfortunately this seems to be the best we can do.

This also improves the ergonomics of inserting a FractionalIndex, by implementing Deref.

This adds tests using sqlx and sqlite, which double as a demonstration of a usage example.

[sergiomeneses]

What is AsRow?

[paulgb]

What is AsRow?

Sorry, I mean FromRow