[dev.fuzz] internal/fuzz: send inputs to workers with shared memory

[dev.fuzz] internal/fuzz: send inputs to workers with shared memory

The coordinator process creates a temporary file for each worker. Both
coordinator and worker map the file into memory and use it for input
values. Access is synchronized with RPC over pipes.

Change-Id: I43c10d7291a8760a616b472d11c017a3a7bb19cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/263153
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Jay Conrod <jayconrod@google.com>