jsonb_delete with arrays

Attached is an implantation of jsonb_delete that instead of taking a single
key to remove accepts an array of keys (it still does just keys, so it's
using the - operator, it's not like the path delete function that also
takes an array, but uses a different operator).

In some simple testing of working through a real world usecases where we
needed to delete 7 keys from jsonb data, it shows approximately a 9x
speedup over calling the - operator multiple times. I'm guessing since we
copy a lot less and don't have to re-traverse the structure.

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/