gc: preserve uint8 and byte distinction in errors, import data

gc: preserve uint8 and byte distinction in errors, import data

There is no semantic change here, just better errors.
If a function says it takes a byte, and you pass it an int,
the compiler error now says that you need a byte, not
that you need a uint8.

Groundwork for rune.

R=ken2
CC=golang-dev
https://golang.org/cl/5300042