Fork child processes to perform work on multiple cores

ForkWork is intended for workloads that a master process can partition into independent jobs, each of which will typically take a while to execute (several seconds, or more). Also, the resulting values should not be too massive, since they must be marshalled for transmission back to the master process.

Among the numerous tools for multicore parallelism available in the OCaml ecosystem, ForkWork fits somewhere in between Netmcore and Parmap. It's a bit easier to use than the former, and a bit more flexible than the latter.