Make verbs b,c,o and U work for any array and slice of integer
type including byte and uint8.

Fix a bug that triggers badverb for []uint8 and []byte type
on the slice/array level instead of on each element like for
any other slice or array type.

Add tests that make sure we do not accidentally alter the
behavior of printing []byte for []byte and []uint8 type
if they are used at the top level when formatting with %#v.

name               old time/op  new time/op  delta
SprintfHexBytes-2   177ns ± 2%   176ns ± 2%   ~     (p=0.066 n=48+49)
SprintfBytes-2      330ns ± 1%   329ns ± 1%   ~     (p=0.118 n=45+47)

Fixes #13478

Change-Id: I99328a184973ae219bcc0f69c3978cb1ff462888
Reviewed-on: https://go-review.googlesource.com/20686
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Rob Pike <r@golang.org>