collate/colltab: improve resilience to Unicode version changes.
collate/colltab: improve resilience to Unicode version changes.

Change-Id: I8bfd6df5b2d35c21e379df108daff8d6fa4eaf86
Reviewed-on: https://go-review.googlesource.com/12031
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

npchp110/text,cmars/tools,cmars/tools,golang/text,cmars/tools,cmars/tools