Users of IndexUnicodePropert(ies|y) should not put their hands in the raw map

[eggrobin]

Some of that was uncovered in #681, but the 💩 is really approaching the 🪭 with #699.
For now I will probably fix what breaks, but I want to come back to it to deal with that properly, which means:

• IUP.load should be void for users (and the one that returns a UnicodeMap should be called loadRawMap).
• most of the IUP.getWhatever should delegate the resolution to the IndexUnicodeProperty (which needs beefing up), or be removed altogether (callers can just use the UnicodeProperty).