[SMTChecker] Add test that has an unused mapping

test/libsolidity/smtCheckerTests/types/unused_mapping.sol

@@ -0,0 +1,17 @@
+pragma experimental SMTChecker;
+
+contract C {
+	uint x;
+	uint y;
+	mapping (address => bool) public never_used;
+
+	function inc() public {
+		require(x < 10);
+		require(y < 10);
+
+		if(x == 0) x = 0; // noop state var read
+		x++;
+		y++;
+		assert(y == x);
+	}
+}