[dev.fuzz] internal/fuzz: add mutator for int types

[dev.fuzz] internal/fuzz: add mutator for int types

Assuming this works, will follow up with another CL
that mutates other types.

Change-Id: Id61acaacd56ca41e3be52e400f8f768672313bbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/308169
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>