runes: added ReplaceIllFormed

This also doubles as a UTF-8 "encoder" and will likely be used as
the decoder in the respective encoding packages.

Change-Id: I9ce0d5fc4c8b458fc1764ed914eab43bfe297081
Reviewed-on: https://go-review.googlesource.com/17707
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
2 files changed