Support conversion to/from Mercury types and JSON.

src/json.m:
    Add two new functions, json.from_type/1 and json.to_type/1.
    The former converts arbitrary Mercury terms (subject to some
    restrictions) to a JSON representation.  The latter processes
    that representation and reconstructs the Mercury term.

src/json.marshal.m:
src/json.unmarshal.m:
    Two new modules that contain the implementations of the new
    functionality.