[dev.fuzz] internal/fuzz: add additional debug logging

When GODEBUG=fuzzdebug=1, log additional debug level information about
what the fuzzer is doing. This provides useful information for
investigating the operation and performance of the fuzzing engine, and
is necessary for profiling new fuzzing strategies.

Change-Id: Ic3e24e7a128781377e62785767a218811c3c2030
Reviewed-on: https://go-review.googlesource.com/c/go/+/324972


Trust: Roland Shoemaker <roland@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>