Fix a typo.

src/json.unmarshal.m:
	As above.
Add a convenience function to assist with building JSON values.

src/json.m:
	Add the function int/1 which returns an integer as a JSON value.
Add utility procedures.

src/json.m:
	Add a bunch of utility procedures for working with JSON values
	and objects.
Fix a bug in the parsing of number literals.

src/json/json.lexer.m:
	A JSON number literal may have an exponent after its fractional part.

	Fix the quoting style used in a spot.

tests/num7.{json,exp}:
tests/num8.{json,exp}:
	Test for the problem with number literals.

tests/bad_hex_digits.exp:
	Conform to the above change to quoting style.