[step_rel] and [iter] theorems

Finally I managed to prove transitivity of [iter]! Unfortunately the
proof is a mess.
Work on a simple stack machine in brainfuck

Currently 4 operations are supported:
- Push
- Addition,
- Subtraction, and
- Dup

I want to implement multiplication as well, preferably by using the
existing operations (I'm looking at you, [add])! The lack of registers
makes it a bit more challenging.