derive Clone, PartialEq and Eq for postgres_types::Json
derive Clone, PartialEq and Eq for postgres_types::Json

It's a bit unwieldy using the `Json` type in structs without this.
`Json` is 'just data', so I think it's usually appropriate to consider
instances to be cloneable and testable for equivalence.

sfackler/rust-postgres,sfackler/rust-postgres