scan() with boundary control

[tomijaga]

Current

btree.insert("a", 1);
btree.insert("alphabet", 2);
btree.insert("b", 3);

let iter = btree.scan(?"a", ?"alphabet");

// iter -> ("a", 1), ("alphabet", 2)

Proposed change

Suppose I wanted to search for all words between "a" and "b" but didn't know the last word starting with "a" was alphabet, I wouldn't be able to do that with the existing scan() method. Enter scanBoundary():

type Boundary<K> = {
    #Inclusive: K;
    #Exclusive: K;
};

let iter = btree.scanBoundary(?#Inclusive("a"), ?#Exclusive("b"));

// iter -> ("a", 1), ("alphabet", 2)