Add From impls for Json

[dbeckwith]

Implements conversion traits between Json and its inner value. Useful for generic code. I ran into this today while trying to return an impl Into<Foo> using sqlx::types::Json.

[dbeckwith]

Hmm I didn't realize that impl<T> From<Json<T>> for T violates some orphan rule. I tried impl<T> Into<T> for Json<T> but that conflicts with the blanket impl. Guess I can't actually get the impl I wanted, but the other one could still be useful.

[dbeckwith]

Latest failures seem unrelated to my changes.

[abonander]

@dbeckwith if you rebase it should fix the failures.