Add locale-sensitive ASCII transliteration API

Add a new function, g_str_to_ascii() that does locale-dependent ASCII
transliteration of UTF-8 strings.

This function works off of an internal database.  We get the data out of
the localedata shipped with glibc, which seems to be just about the best
source of locale-sensitive transliteration information available
anywhere.

We include a update script with this commit that's not used by anything
at all -- it will just sit in git.  It is intended to be run manually
from time to time.

https://bugzilla.gnome.org/show_bug.cgi?id=710142