mime/multipart: convert Reader from interface to struct

It was always a weird interface but I didn't know what I
was doing at the time.  rsc questioned me about it then
but didn't press on it during review.  Then adg bugged me
about it too recently.

So clean it up. It parallels the Writer struct too.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/4602063
4 files changed