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

test/libsolidity/syntaxTests/receiveEther/old_syntax.sol

@@ -0,0 +1,5 @@
+contract C {
+    function() external payable {}
+}
+// ----
+// ParserError: (45-46): Expected a state variable declaration. If you intended this as a fallback function or a function to handle plain ether transactions, use the "fallback" keyword or the "ether" keyword instead.