[io grant] Implement IO::Path.concat-with

- Does what .child used to do
- .child will be changed to throw for non-child paths

src/core/IO/Path.pm

@@ -329,6 +329,10 @@ my class IO::Path is Cool {
         );
     }
 
+    method concat-with (IO::Path:D: Str() \what) {
+        self.bless: :path($!SPEC.join: '', $!path, what), :$!SPEC, :$!CWD;
+    }
+
     proto method chdir(|) { * }
     multi method chdir(IO::Path:D: Str() $path, :$test!) {
         DEPRECATED(