[dev.fuzz] internal/fuzz: implement basic mutator and use in worker

[dev.fuzz] internal/fuzz: implement basic mutator and use in worker

Note that this is an extremely basic mutator, and only
meant to act as a placeholder for future development.

Change-Id: I650691db44f30953345702aac93cbd1cadc21427
Reviewed-on: https://go-review.googlesource.com/c/go/+/263657
Trust: Katie Hockman <katie@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>