exp/norm: a few minor fixes to support the implementation of norm.

exp/norm: a few minor fixes to support the implementation of norm.
maketables.go/tables.go
- Properly set combinesForward flag for JamoL and JamoV.
- Fixed Printf bug.
composition.go
- Make insertString use the same control flow as insert.
- Better Hangul and non-Hangul mixing.
forminfo.go
- Fixed bug in compBoundaryBefore that affected a few esoteric cases.
- Buffer overflow now tested in normalize_test.go (other CL).

R=r
CC=golang-dev
https://golang.org/cl/4924041