Function upper_bound_from_prefix 

pub fn upper_bound_from_prefix(prefix: &str) -> Option<String>

Find the key to use as an upper bound for range query

let mut data: BTreeMap<String, i32> = BTreeMap::new();
data.insert("bbabb".to_owned(), 1);
data.insert("bbbcc".to_owned(), 2);
data.insert("bbbab".to_owned(), 3);
data.insert("bbb".to_owned(), 4);
data.insert("bbc".to_owned(), 5);

let lower = Bound::Included("bbb".to_owned());
let upper = Bound::Excluded(prefix_range::upper_bound_from_prefix("bbb").unwrap());
let in_prefix: Vec<_> = data.range((lower, upper)).map(|(_, v)| *v).collect();
assert_eq!(in_prefix, vec![4, 3, 2]);