Features: insert_or_get_mut()#38

Hi, it would be nice if a `insert_or_get_mut()` would exist. my use case: The Qp tries store bitmaps. I have a new bitmap `A` associated with the key `1`. I would like the perform a binary and operation if a bitmap `B` for key `1` is already in the trie otherwise I would like to insert `A`. So that the value of key `1` is now `A & B`. Currently I simple call `.insert(A)`. If I get `Some` as return value I perform `insert(A & B)` again. But so I must run twice through the trie.