Add BTreeMap::remove_entry
Add BTreeMap::remove_entry

Mainly for API parity with HashMap.

- Add BTreeMap::remove_entry
- Rewrite BTreeMap::remove to use remove_entry
- Use btreemap_remove_entry feature in doc comment

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust