all: use strings.Builder instead of bytes.Buffer where appropriate

I grepped for "bytes.Buffer" and "buf.String" and mostly ignored test
files. I skipped a few on purpose and probably missed a few others,
but otherwise I think this should be most of them.

Updates #18990

Change-Id: I5a6ae4296b87b416d8da02d7bfaf981d8cc14774
Reviewed-on: https://go-review.googlesource.com/102479
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -376,7 +376,7 @@ func (c *common) decorate(s string) string {
 		file = "???"
 		line = 1
 	}
-	buf := new(bytes.Buffer)
+	buf := new(strings.Builder)
 	// Every line is indented at least one tab.
 	buf.WriteByte('\t')
 	fmt.Fprintf(buf, "%s:%d: ", file, line)