insert_overwrite should return results from cut

[stefnotch]

I would have expected insert_overwrite to return an iterator over the old items, since most other insert methods return the old values or something.

Judging from the source code, it'd be a quick tweak.
https://docs.rs/discrete_range_map/latest/src/discrete_range_map/discrete_range_map.rs.html#1232-1240

[ripytide]

Good point, I didn't need this in my use-case, but that would match up with the BTreeMap's insert. The other insert_* methods are are bit trickier in this regard so might require a bit more work.