Change opcode generated by (panic) to INVALID

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.

liblll/CodeFragment.cpp

@@ -575,7 +575,7 @@ void CodeFragment::constructOperation(sp::utree const& _t, CompilerState& _s)
 		}
 		else if (us == "PANIC")
 		{
-			m_asm.appendJump(m_asm.errorTag());
+			m_asm.append(Instruction::INVALID);
 		}
 		else if (us == "BYTECODESIZE")
 		{