Add more back/front methods and deprecate unspecified inserts

[cuviper]

For RingMap and RingSet, this just deprecates insert_full in favor of the existing push_back and push_front. The plain insert method is left as-is, as a sort of "I don't really care about order" method that doesn't return an index, but it also documents to see the explicit push methods.

For Entry types, I've added a bunch of back/front methods and deprecated all the insert-like originals.

• insert => push_back, push_front
• insert_entry => push_back_entry, push_front_entry
• or_default => or_push_back_default, or_push_front_default
• or_insert => or_push_back, or_push_front
• or_insert_with => or_push_back_with, or_push_front_with
• or_insert_with_key => or_push_back_with_key, or_push_front_with_key

Fixes #10.