Include column numbers in all reader error contexts.

src/json.m:
	Add a impure mutable reference to the reader type
	for storing column numbers.  Our usage of this is
	pure since it is effectively attached to the stream
	state.

	Include columns numbers in error messages.

src/json.json_lexer.m:
	Update column numbers where necessary.

src/json.json_parser.m:
tests/*.exp:
	Conform to the above changes.