mogrify and indent features for jsonb

Attached is a patch to provide a number of very useful facilities to
jsonb that people have asked for. These are based on work by Dmitry
Dolgov in his jsonbx extension, but I take responsibility for any bugs.

The facilities are:

new operations:

concatenation: jsonb || jsonb -> jsonb
deletion: jsonb - text -> jsonb
deletion: jsonb - int -> text

new functions:

produce indented text: jsonb_indent(jsonb) -> text
change an element at a path: jsonb_replace(jsonb, text[], jsonb) -> jsonb.

It would be relatively trivial to add:

delete an element at a path: jsonb_delete(jsonb, text[]) -> json

and I think we should do that for the sake of completeness.

The docs might need a little extra work, and the indent code definitely
needs work, which I hope to complete in the next day or two, but I
wanted to put a stake in the ground.

cheers

andrew