undo CL 8363045 / a3ce42f9748b

It changes an exported API, and breaks the build.

??? original CL description
reflect: use unsafe.Pointer in StringHeader and SliceHeader

Relates to issue 5193.

R=r
CC=golang-dev
https://codereview.appspot.com/8363045
???

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8357051