archive/tar: Make Reader and Writer more closely follow io.Reader and io.Writer inter...

archive/tar: Make Reader and Writer more closely follow io.Reader and io.Writer interfaces

There's no functional change here. io gives the Read and Write methods byte slice arguments, but tar called them uint8. It's the same thing, but I think this is clearer and it matches what other packages do.

R=rsc
CC=golang-dev
https://golang.org/cl/157099