Unicode::UCD::prop_invmap(): New improved API

Unicode::UCD::prop_invmap(): New improved API

Thanks to Tony Cook for suggesting this.

The API is changed from returning deltas of code points, to storing the
actual correct values, but requiring adjustments for the non-initial
elements in a range, as explained in the pod.

This makes the data less confusing to look at, and gets rid of
inconsistencies if we didn't make the same sort of deltas for entries
that were, e.g. arrays of code points.