Merge pull request #4139 from ethereum/abiEncodeIsPure

ABI encoding functions are pure and should be usable in constants.

test/libsolidity/syntaxTests/constants/abi_encoding_constant.sol

@@ -0,0 +1,7 @@
+contract C {
+    bytes32 constant a = keccak256(abi.encode(1, 2));
+    bytes32 constant b = keccak256(abi.encodePacked(uint(1), a));
+    bytes32 constant c = keccak256(abi.encodeWithSelector(0x12345678, b, 2));
+    bytes32 constant d = keccak256(abi.encodeWithSignature("f()", 1, 2));
+}
+// ----