Derive transparent

[malobre]

#[postgres(transparent)]

Acknowledgments

• @nolanderc for the original PR: Added support for NewType/transparent structs sfackler/rust-postgres-derive#18

Example

#[derive(ToSql, FromSql)]
#[postgres(transparent)]
struct UserId(i32);

[sfackler]

Nice! Would also be good to have a few quick tests.

[malobre]

I added basic tests and updated some of the logic.

[sfackler]

Can you add a little bit of docs to the postgres-types crate?

LTGM otherwise!

[malobre]

I added documentation and squashed all the commits into one.
Edit: Had to squash again, I missed one of your comment.