Run Libpostal as child process

[missinglink]

This is very much inspired by the PR to use libpostal over an HTTP API and the great thing is it uses the same async/await API. So all methods are interchangeable, be it sync, async, child_process or http.

This PR moves the loading of libpostal to a child module and creates a message bus that can be shared in a multi-threaded environment.

[orangejulius]

Interesting! Does this have any advantages over using HTTP?

[missinglink]

Yet to be determined, it certainly wasn't the silver bullet I was hoping it would be.