Fix some typos in comments.

src/string_reader.m:
	As above.
Initial import of the tee writer module.

src/tee.m:
	A module that provides tee (as in the Unix command) writer streams.

src/mercury_misc.m
	Import the new module.

src/string_reader.m:
	Adjust some comments.

LICENSE:
	Bump the copyright year.