Separate out custom binary search

The binary search done in `next()` and `next_back()` is basically just a regular `slice::binary_search_by` with added access to the current index in the function. The two code chunks are nearly identical and can be separate out into a separate `enumerated_binary_search_by()` method.

This could either be a helper method in this crate or a separate crate on its own. That depends on whether we think people will use it in other contexts, I suppose.