The old code's structure needed to track indirections because of the
use of unsafe. That is no longer necessary, so we can remove all
that tracking. The code cleans up considerably but is a little slower.
We may be able to recover that performance drop. I believe the
code quality improvement is worthwhile regardless.

BenchmarkEndToEndPipe           5610          5780          +3.03%
BenchmarkEndToEndByteBuffer     3156          3222          +2.09%

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/103700043