all: prefer bytes.IndexByte over bytes.Index

bytes.IndexByte can be used wherever the second argument to
strings.Index is exactly one byte long, so we do that with this change.

This avoids generating unnecessary string symbols/converison and saves
a few calls to bytes.Index.

Change-Id: If31c775790e01edfece1169e398ad6a754fb4428
Reviewed-on: https://go-review.googlesource.com/66373
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/json/decode_test.go

@@ -88,7 +88,7 @@ func (u unmarshalerText) MarshalText() ([]byte, error) {
 }
 
 func (u *unmarshalerText) UnmarshalText(b []byte) error {
-	pos := bytes.Index(b, []byte(":"))
+	pos := bytes.IndexByte(b, ':')
 	if pos == -1 {
 		return errors.New("missing separator")
 	}