utf8.c: Use proper Unicode property names

utf8.c: Use proper Unicode property names

There are five functions in utf8.c that look up Unicode maps--the case
changing functions.  They look up these maps under the names ToDigit,
ToFold, ToLower, ToTitle, and ToUpper.  The imminent expansion of Unicode::UCD
to return the mappings for all properties creates a naming conflict, as
three of those names are the same as other properties, Upper, Lower, and
Title.

It was an unfortunate choice of names originally.  Now mktables has been
changed to create a list of mapping properties that utf8_heavy.pl reads.
It uses the official names of those properties, so change utf8.c to
correspond.