strings: add benchmark for Builder.WriteString

strings: add benchmark for Builder.WriteString

This is in preparation for an optimization.

```
name                                     time/op
BuildString_WriteString/1Write_NoGrow-8  30.8ns ± 0%
BuildString_WriteString/3Write_NoGrow-8   111ns ± 0%
BuildString_WriteString/3Write_Grow-8    44.1ns ± 4%

name                                     alloc/op
BuildString_WriteString/1Write_NoGrow-8   48.0B ± 0%
BuildString_WriteString/3Write_NoGrow-8    336B ± 0%
BuildString_WriteString/3Write_Grow-8      112B ± 0%

name                                     allocs/op
BuildString_WriteString/1Write_NoGrow-8    1.00 ± 0%
BuildString_WriteString/3Write_NoGrow-8    3.00 ± 0%
BuildString_WriteString/3Write_Grow-8      1.00 ± 0%
```

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I0e2c82edcdc72f381c5160f315401678ff76f3d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/507777
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>