This change adds support for decoding and encoding the bool type. The
encoding is a single byte, with a zero value for false and a non-zero
value for true.

Closes #16856.

Change-Id: I1d1114b320263691473bb100cad0f380e0204186
Reviewed-on: https://go-review.googlesource.com/28514Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>