[dev.fuzz] internal/fuzz: stablize mutator benchmark and add additional benchmarks

Adds a few new benchmarks, and attempts to reduce the variability of the
existing BenchmarkMutatorBytes benchmark. These should help provide some
insight when we're working on performance issues.

Change-Id: I45b68ae36da99ec2eb4a610b7a3fc6fbf3d9494a
Reviewed-on: https://go-review.googlesource.com/c/go/+/324969


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