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 .
No. `push_back` takes a container and an element of the container's value type, and pushes the element back into the container. *Every* `push_back` member in the entire standard library takes a single value.