simulation: add mutation (disabled) and fix bf interpreter

So I was using safe subtraction to fix the heads at 0, but BF itself
wraps heads around the tape if they overflow/underflow.  This
completely changes the presentation of the simulation.

src/simulation.h

@@ -13,7 +13,6 @@
 
 typedef struct
 {
-  u64 a, b;
   bf_token memory[SIMULATION_SIZE];
 } simulation_t;