Add support for Numeric to SQLite

[sgrif]

As far as I can tell, there is no "internal" representation of this that we can deserialize to (like we did with PgNumeric). As such, we should probably have this deserialize to a string, as that's the only type that we can reasonably implement FromSql for without losing precision. We can add ToSql impls for f32 and f64 and/or the various integer types if we think it's useful enough.

[sgrif]

Upon further investigation, SQLite appears to have no visible difference in behavior between numeric and double precision floats. If anybody knows this to be incorrect, please let me know.