An API to find the closest interval to a value

It would be great to extend Lapper's modus operandi to also perform the "fuzzy" search - instead of the exact match inside one of the intervals, allow to search for the closest interval when there is no exact match. 

The API could be similar to `find`:
```rust
pub fn closest(&self, start: I, stop: I) -> IterFind<'_, I, T>ⓘ
```