Removed bytes.Add and bytes.AddByte; we now have 'append'.
Changed all uses of bytes.Add (aside from those testing bytes.Add) to append(a, b...).
Also ran "gofmt -s" and made use of copy([]byte, string) in the fasta benchmark.

R=golang-dev, r, r2
CC=golang-dev
https://golang.org/cl/3302042