This is an XS wrapper around some Unicode Consortium code to check if a string
is valid UTF-8, revised to conform to what expat/Mozilla think is valid UTF-8,
especially with regard to low-ASCII characters.
Note that this module has NOTHING to do with Perl's internal UTF8 flag on
scalars.
This module is for use when you're getting input from users and want to make
sure it's valid UTF-8 before continuing.
