Split fallback function and introduce "fallback()" and "receive()" syntax.

test/libsolidity/gasTests/dispatch_small_optimised.sol

@@ -2,7 +2,7 @@ contract Small {
     uint public a;
     uint[] public b;
     function f1(uint x) public returns (uint) { a = x; b[uint8(msg.data[0])] = x; }
-    function () external payable {}
+    fallback () external payable {}
 }
 // ====
 // optimize: true