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

src/pkg/regexp/regexp.go

@@ -675,7 +675,7 @@ Loop:
 			break Loop
 		}
 		n := utf8.EncodeRune(utf, inst.(*_Char).char)
-		b = bytes.Add(b, utf[0:n])
+		b = append(b, utf[0:n]...)
 		i = inst.next().index()
 	}
 	// point prefixStart instruction to first non-CHAR after prefix