Add stack implementation.

Add:
	makefile
		-Replace `make wildcard` source file matching with a call to the `find`
		utility; obviates relying on shell-specific wildcard expansion and
		manual subdirectory listing.

	src/interpreter/stack/
		stack.h
			-Wrapper macros for `single_linked_list.h`'s functions that mimic
			LIFO stack behavior.

		single_linked_list.(c, h)
			-An implementation of a singly-linked list; used to implement
			`stack.h`.