Allow ABI encoding for array slices without explicit casts.

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

@@ -0,0 +1,8 @@
+contract c {
+  bytes public b;
+  function f() public {
+    b = msg.data[:];
+  }
+}
+// ----
+// TypeError: (63-74): Type bytes calldata slice is not implicitly convertible to expected type bytes storage ref.

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

@@ -4,4 +4,3 @@ contract C {
     }
 }
 // ----
-// TypeError: (85-91): This type cannot be encoded.