Provide more standard functions

[leostera]

I'd like to keep track of some standard functions that we will need to build up more general patterns a la OTP.

Processes

• [ ] exit : (Pid.t, ~reason:string) => unit — force termination of an actor.
• [ ] is_alive : Pid.t => bool — whether a Pid is alive or not
• [ ] register : (Pid.t, ~name:string) => unit — register a Pid with a well-known name
• [ ] unregister (~name:string) => unit — unregisters a process if it's registered.
• [ ] where_is : (~name:string) => option(Pid.t) — find a pid registered with a name
• [ ] registered : unit => list((Pid.t, string)) — returns a list of tuples with the Pid and name they were registered with

System

• [x] halt : unit => unit — notify the main scheduler we should take the system down.