A Rust version for fractional-indexing . Keys in string and satisfy lexicographic order.
Rewritten from existed codebase:
Docs.
```rs use lexiconfractionalindex::key_between;
// create an initial key let k0 = key_between(None, None)?;
// two keys, notice that some strings are not valid keys let left = "Xb0M".toowned(); let right = "Xb0M0V".toowned();
// new key at beginning let next = key_between(None, Some(&right))?;
// new key at end let next = key_between(Some(&left), None)?;
// new key between 2 keys let next = key_between(Some(&left), Some(&right))?; ```
TODO