Add new JSON processing functions and parser API.
 
The JSON parser is converted into a recursive descent parser, and
exposed for use by other modules such as extensions. The API provides
hooks for all the significant parser event such as the beginning and end
of objects and arrays, and providing functions to handle these hooks
allows for fairly simple construction of a wide variety of JSON
processing functions. A set of new basic processing functions and
operators is also added, which use this API, including operations to
extract array elements, object fields, get the length of arrays and the
set of keys of a field, deconstruct an object into a set of key/value
pairs, and create records from JSON objects and arrays of objects.
 
Catalog version bumped.
 
Andrew Dunstan, with some documentation assistance from Merlin Moncure.

Like the XML data type, we simply store JSON data as text, after checking
that it is valid.  More complex operations such as canonicalization and
comparison may come later, but this is enough for not.
 
There are a few open issues here, such as whether we should attempt to
detect UTF-8 surrogate pairs represented as \uXXXX\uYYYY, but this gets
the basic framework in place.

Also move the escape_json function from explain.c to json.c where it
seems to belong.
 
Andrew Dunstan, Reviewd by Abhijit Menon-Sen.