fmt: unify array and slice formatting for bytes and other types
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>