exp/norm: fixed two unrelated bugs in normalization library.

exp/norm: fixed two unrelated bugs in normalization library.
1) incorrect length given for out buffer in String.
2) patchTail bug that could cause characters to be lost
   when crossing into the out-buffer boundary.

Added tests to expose these bugs.  Also slightly improved
performance of Bytes() and String() by sharing the reorderBuffer
across operations.

Fixes #2567.

R=r
CC=golang-dev
https://golang.org/cl/5502069