json & jsonb constraints support

[marocchino]

Hi, I want some constraints to json just like this.

Unfortunately, current version can not receive something like jsontable(((j->>'uuid')::uuid)) because of this wrapper.

https://github.com/nullobject/rein/blob/f5298945754aa7dee9bd142218208dcb2164a16a/lib/rein/util.rb#L17-L21

If you don't mind, I want make PR for it. how do you think?

[nullobject]

Sure, if you're happy to make a PR please go ahead 😄