by other low-level libraries, like collate.  Extra care has been given to optimize the performance
of normalizing to NFD, as this is what will be used by the collator.  The overhead of checking
whether a string is normalized vs simply decomposing a string is neglible.  Assuming that most
strings are in the FCD form, this iterator can be used to decompose strings and normalize with
minimal overhead.

R=r
CC=golang-dev
https://golang.org/cl/5676057