Introduce a non-recursive JSON parser

Introduce a non-recursive JSON parser

This parser uses an explicit prediction stack, unlike the present
recursive descent parser where the parser state is represented on the
call stack. This difference makes the new parser suitable for use in
incremental parsing of huge JSON documents that cannot be conveniently
handled piece-wise by the recursive descent parser. One potential use
for this will be in parsing large backup manifests associated with
incremental backups.

Because this parser is somewhat slower than the recursive descent
parser, it  is not replacing that parser, but is an additional parser
available to callers.

For testing purposes, if the build is done with -DFORCE_JSON_PSTACK, all
JSON parsing is done with the non-recursive parser, in which case only
trivial regression differences in error messages should be observed.

Author: Andrew Dunstan
Reviewed-By: Jacob Champion
Discussion: https://postgr.es/m/7b0a51d6-0d9d-7366-3a1a-f74397a02f55@dunslane.net