unicode/bidi: moved bidi package to unicode

This is in line with the other recent moves.

BiDi is inherently low-level Unicode standard and as such belongs
in that directory.

Change-Id: I992560fdb4f08cae3152611154c68af713f54652
Reviewed-on: https://go-review.googlesource.com/17781
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
11 files changed