added BL instruction (Branch with Link, a subroutine-call that saves the return address in the new register LINK).
added RET instruction (technically a MOV from LINK register to PC, but we don't have MOV yet), to be able to return from subroutine calls.

To be able to do multi-level subroutines (subroutines calling other subroutines), at the very least I need to add a way to directly access the LINK register.