Initial introduction of array slices with partial implementation for dynamic calldata arrays.

test/libsolidity/syntaxTests/array/slice/calldata_dynamic_forward.sol

@@ -0,0 +1,7 @@
+contract C {
+    function f(bytes calldata x) external {
+        return this.f(x[1:2]);
+    }
+}
+// ----
+// TypeError: (79-85): Invalid type for argument in function call. Invalid implicit conversion from bytes calldata slice to bytes memory requested.