We can now "call" subroutines, and let the stack handle it.

The following code works as expected:

        store #1, 15
        call print

        store #1, 240
        call print

        store #1, 255
        call print

        exit

    :print
        print_int #1
        store #1, "\n"
        print_str #1
        ret

Each time a `call` instruction is seen we store the next address
on the stack, which the `ret` instruction will pop-off and jump
to.