Refactor deepContains(), and move to jsonb_support.c

Tweak the deepContains() implementation. Add extensive comments where
before there were very few.

The new location seems more appropriate, and allows jsonb_ops.c to
consist only of client functions to what could broadly be classified as
"walker infrastructure" that now all lives in jsonb_support.c.

A few other jsonb_support.c tweaks are made in passing.