Add `stateMutability field to JSON AST for address` types.

test/libsolidity/ASTJSON/address_payable.sol

@@ -0,0 +1,11 @@
+contract C {
+    mapping(address => address payable) public m;
+    function f(address payable arg) public returns (address payable r) {
+        address payable a = m[arg];
+        r = arg;
+        address c = address(this);
+        m[c] = address(0);
+    }
+}
+
+// ----