fmtBytes is in the top 10 callers of runtime.slicebytetostring according
to Google wide profiling data.

Avoid the string conversion of the input byte slice in fmtBytes by calling
a newly added specialized fmtS function for byte slices.

Expand tests for verb s with widths to test strings and byte slice arguments.

SprintfTruncateString     157ns ± 4%     156ns ± 3%     ~     (p=0.122 n=20+20)
SprintfTruncateBytes      188ns ± 2%     155ns ± 3%  -18.00%  (p=0.000 n=20+19)

name                   old alloc/op   new alloc/op   delta
SprintfTruncateString     16.0B ± 0%     16.0B ± 0%     ~     (all equal)
SprintfTruncateBytes      64.0B ± 0%     16.0B ± 0%  -75.00%  (p=0.000 n=20+20)

name                   old allocs/op  new allocs/op  delta
SprintfTruncateString      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
SprintfTruncateBytes       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=20+20)

Change-Id: I461bf514d4232b39bd9c812f7faa4e5ef693a03b
Reviewed-on: https://go-review.googlesource.com/c/145284
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>