When PR #2317 changed the EVM opcode from SHA3 to KECCAK256 it broke the `(sha3 loc len)` expression in LLL. This PR fixes things while allowing existing code using the sha3 expression (such as the ENS registrar) to continue to compile. I.e. both `(keccak256 loc len)` and `(sha3 loc len)` may be used, and the existing related sha3 macros continue to work.
Three end-to-end test cases have been added for kekkac256 and sha3.
EIP-141 ethereum/EIPs#141 has preserved 0xfe as an invalid opcode for aborting EVM execution. The EVM assembler supports this via the INVALID opcode.
The LLL "panic" expression used to generate a jump to an invalid location in order to abort EVM execution. This change brings "panic" into line with EIP-141 by generating the INVALID opcode instead.
