insert/push_back/push_front range extension

Add range-based non-member non-friend insert extension, push_back/push_front utilities that build upon it, and RANGE_RETURNS utility macro.

Closes #4 .
> [`Sorted`] is also a lot of work to get it right, and the ROI on implementing it is not clear at all.

I agree with this. I've never been a fan of this idea to surface sorted-ness in the type system. It's too much nannying for my taste.