Manually implement Debug for BTreeMap::ValuesMut struct
Manually implement Debug for BTreeMap::ValuesMut struct

Deriving debug prints all the values including keys. But ValuesMut
struct should only print the values.

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