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

test/libsolidity/semanticTests/fallback/short_data_calls_fallback.sol

@@ -0,0 +1,17 @@
+contract A {
+    uint public x;
+    // Signature is d88e0b00
+    function fow() public { x = 3; }
+    fallback () external { x = 2; }
+}
+// ----
+// (): hex"d88e0b"
+// x() -> 2
+// (): hex"d88e0b00"
+// x() -> 3
+// (): hex"d88e"
+// x() -> 2
+// (): hex"d88e0b00"
+// x() -> 3
+// (): hex"d8"
+// x() -> 2