Use RangeBounds<usize> instead of usize.

The parameters for the `substring` method can be changed to a RangeBounds. This will allow for providing inclusive end points, as well as providing unbounded start/end points (as has been requested in #5).

This would be a breaking change, but I think it will make the library a bit easier to use. I would rather do this than introduce several methods for each specific case. It will also raise MSRV to match what is required for `RangeBounds`.