Fix bug in prefixing scheme

Fix bug in prefixing scheme

As Bawolff pointed out at [[mw:User talk:Simetrical/Collation]], the
prefixing scheme I was using meant that the page "Z" with sort key of
"F" would sort after a page named "A" with a sort key of "FF", since the
first one's raw sort key would compute to "FZ", and the second's would
compute to "FFA".  I've fixed this by separating the prefix from the
unprefixed part by a null byte (cl_sortkey is eventually going to be
totally binary anyway, may as well start now).