Correct the type signature of `insert`
Correct the type signature of `insert`

We were relying on the fact that `Table` always ends up implementing
`AsQuery`, and our `SqlType` for that is the same as the SqlType of
`Star`. However, this isn't technically correct, and can be painful when
abstracting over a table in multiple contexts involving `Star`

mcasper/diesel,diesel-rs/diesel,sgrif/diesel,mcasper/diesel,sgrif/diesel,cascalheira/diesel,tohou/diesel,tohou/diesel,SoftwareWarlock/diesel,cascalheira/diesel,diesel-rs/diesel,SoftwareWarlock/diesel