[S15] Clarify values the uniprops hash can contain

Based off the value types defined in UAX#44, §5.2, the mapping is now
thus:

    B → Bool
    N → Rat
    C, E, S, M → Str

Though perhaps the 'E'numeration types could actually be Enums, if the
benefits of codifying them as enumerations is worth it.
[S15] Modify "Final Considerations" on mismatching in operators.

Essentially, NFC ~ NFD -> Uni, instead of the previous "causes an error"
definition. Note that detailing various operators and functions still
needs doing.