[Rust] Add support for binary search returning the index

[paraseba]

Sometimes finding the position of an object in a vector is more important than finding the object itself. Vectors currently have lookup_by_key, but that can only return the object, not the index.

Can we introduce a new function that returns the index instead?

Why are indexes important? You could have a data structure that uses vector indices as "pointers" into the vector. In this situation, calculating after you search for an element you may need its index. There are many other examples.