allow symbolic bytecode in hevm equivalence

[charles-cooper]

in runtime bytecode, there is often bytecode which is never executed, but may be copied to memory via codecopy. it would be nice to be able to have bytecode like

CODECOPY 0, 100, 32

and in the actual bytecode provided to hevm equivalence, use sentinel nibbles like SSSS..SS at offset 100. then hevm would interpret this as symbolic data. of course, if the symbolic bytecode is actually reachable (it shouldn't be!), execution should terminate with an exception.