fix: sha3

sha3 returns a string of bytes, but MSTORE expects something that can be divided by 256.  So a contract like 'a=sha3(1); return(a)' was failing when the result of sha3 was to be loaded form the stack into memory.  So we convert to int.

In sha3 itself, rlp.decode was given two arguments but it only takes one.  It was also returning weird results that truncated the hash, so I just removed it altogether (so the raw bytes returned by sha3 are added to the stack).

Not sure if I missed something but this was working for me.