.cast() as an operator

[Mike Bayer]

Looking at https://wiki.postgresql.org/images/b/b4/Pg-as-nosql-pgday-fosdem-2013.pdf I'm reminded that when we add the PG JSON type, we're going to want to pull out fields and cast them in PG's style, i.e. "data::type". Some TODOs:

1. add cast() as an operator, that on most backends produces CAST()

2. on PG, it produces the "::" format.

3. add a coerce() operator as well, that does type_coerce().

4. docs for cast(), type_coerce(), and the operators should all cross-link each other

[Mike Bayer]

this is pretty specific to the new JSON type for now. 2104d0ba2d612a26d36

[Mike Bayer]

Removing milestone: 0.9.0 (automated comment)