Surrogate halves are part of UTF-16 and should never appear in UTF-8.
(The rune that two combined halves represent in UTF-16 should
be encoded directly.)

Encoding: encode as RuneError.
Decoding: convert to RuneError, consume one byte.

This requires changing:
        package unicode/utf8
        runtime for range over string
Also added utf8.ValidRune and fixed bug in utf.RuneLen.

Fixes #3927.

R=golang-dev, rsc, bsiegert
CC=golang-dev
https://golang.org/cl/6458099