radix.go
func (t *Tree) DeletePrefix(s string) int {
	return t.deletePrefix(nil, t.root, s)
}
func (t *Tree) DeletePrefix(s string) int {
	return t.deletePrefix(nil, t.root, s)
}
// DeletePrefix is used to delete the subtree under a prefix
// Returns how many nodes were deleted
// Use this to delete large subtrees efficiently