Public tests for part 4:

This part tests your typechecking implementation on a number
of sample rube programs.

Tests:
	r1.ru -- typechecks
	r2.ru -- typechecks
	r3.ru -- typechecks
	r4.ru -- typechecks
	r5.ru -- typechecks
	cast.ru -- typechecks
	badcast.ru -- doesn't typecheck
	objnil.ru -- typechecks
