strings: optimize Replace by using a strings.Builder

name        old time/op    new time/op    delta
ReplaceAll     162ns ±26%     134ns ±26%  -17.44%  (p=0.014 n=10+10)

name        old alloc/op   new alloc/op   delta
ReplaceAll     32.0B ± 0%     16.0B ± 0%  -50.00%  (p=0.000 n=10+10)

name        old allocs/op  new allocs/op  delta
ReplaceAll      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)

Change-Id: Ia8377141d3adb84c7bd94e511ac8f739915aeb40
Reviewed-on: https://go-review.googlesource.com/c/go/+/245197
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/strings/strings_test.go

@@ -1900,3 +1900,12 @@ func BenchmarkTrimSpace(b *testing.B) {
 		})
 	}
 }
+
+var stringSink string
+
+func BenchmarkReplaceAll(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		stringSink = ReplaceAll("banana", "a", "<>")
+	}
+}