Fix memory leaks; remove the utils module.

Rem:
	src/interpreter/utils.(c, h)
		-The module is currently obsolete, because ts file input function was
		used only by the engine's shell and old interpreter, both of which have
		been removed; consequently, remove it.

Fix:
	src/interpreter/interpreter.c
		-Patch up all memory leaks detected by `valgrind`.

	src/interpreter/stack/(single_linked_list, stack).(c, h)
		-Have `freeList()` accept a pointer to a function that deallocates a
		list's data pointers.