io: change the name of ReadAtSizer to SizedReaderAt

io: change the name of ReadAtSizer to SizedReaderAt

This is a proposal. The old name is pretty poor. The new one describes
it better and may be easier to remember. It does not start with Read,
though I think that inconsistency is worthwhile.

Reworded the comment a bit for clarity.

Change-Id: Icb4f9c663cc68958e0363d7ff78a0b29cc521f98
Reviewed-on: https://go-review.googlesource.com/21629
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>