feature: Multi-threaded parallel transformation

[piglovesyou]

In RxJava,

By default, the parallelism level is set to the number of available CPUs

I want to do that in Hole with child_process.fork.

Syntax will be, ?

holeWithArray([])
    .pipe(() => {}, {
        boost: true, // (boolean|number). true to distribute number of CPUs to processes, or number to fix specific number of CPUs.
    })

[piglovesyou]

Considering overhead of IPC, if decent multi-thread functionality exists that would be better. Napa.js might be our solution?