intDataSize should return length of bool slice, so functions
Read and Write can use the fast path to process bool slice.

Change-Id: I8cd275e3ffea82024850662d86caca64bd91bf70
Reviewed-on: https://go-review.googlesource.com/112135Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>