Figure out how to allow querying values against a nullable column

[sgrif]

I'm debating whether or not to have foo.eq(None) result in foo IS NULL or not. Either way, for ergonomics, we should allow foo.eq("bar") in addition to foo.eq(Some("bar")). At the moment, attempting to write this ToSql implementation results in overlap both with the impl for Option<T>, and for &'a T. I think we need specialization for this to work.