bidi: rename BiDi class constants to short Unicode names.

This greatly simplifies porting. The reference implementation
uses similar names.

Change-Id: Idc670c8ea3441258cfb0c9d82e1cbf9026794df6
Reviewed-on: https://go-review.googlesource.com/17293
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
6 files changed