Support `diesel_manage_updated_at` on SQLite

[sgrif]

There's a little bit of funkiness required to make this work. The first
piece is that we need to modify the function registration to provide
access to the connection for our internal functions. This isn't
strictly necessary, we could transmute the &self to be &'static self, since the function will never be called after self is dropped.
That felt like it would add some unneccessary unsafety though.

The second bit of funkiness is that we have to have some return type,
so we can't just return (). I think I want to fix this in the future
by providing a Null SQL type, which provides ToSql and FromSql
impls only for (). This requires adding a variant to SqliteType and
MysqlType though, so it will have to be done in 2.0.

The function itself is subtly different from the PostgreSQL version,
since it runs even if no values were actually changed (we can't do
something like NEW IS DISTINCT FROM OLD here). This also cannot be run
on tables without ROWIDs. I think we should probably declare this as a
SQL function in code somewhere for documentation purposes, but I want to
wait until our docs are building again to follow up with that.