Implement conversion from XID to u32

[msmoiz]

I was trying to deserialize a system table and received the following error:

cannot convert between the Rust type `core::option::Option<u32>` and the Postgres type `xid`

Is it reasonable to implement FromSql for this conversion within the types crate, or is there a good reason to have consumers do it themselves using a wrapper type? Based on the documentation, it seems like XID is 32-bits in PostgreSQL, so I would expect this to be the correct conversion in most cases.

P.S. Thanks for supporting this crate!