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

test/libsolidity/semanticTests/fallback/fallback_or_receive.sol

@@ -0,0 +1,17 @@
+contract C {
+    uint x;
+    uint y;
+    fallback () payable external { ++x; }
+    receive () payable external { ++y; }
+    function f() external returns (uint, uint) { return (x, y); }
+}
+// ----
+// f() -> 0, 0
+// () ->
+// f() -> 0, 1
+// (), 1 ether ->
+// f() -> 0, 2
+// (): 1 ->
+// f() -> 1, 2
+// (), 1 ether: 1 ->
+// f() -> 2, 2

test/libsolidity/semanticTests/fallback/inherited.sol

@@ -0,0 +1,10 @@
+contract A {
+    uint data;
+    fallback() external { data = 1; }
+    function getData() public returns (uint r) { return data; }
+}
+contract B is A {}
+// ----
+// getData() -> 0
+// (): 42 ->
+// getData() -> 1

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