remove ambiguity b/t Supervisor/Child/Starter PIDs

[tavisrudd]

• use SupervisorPid, ChildPid, StarterPid type aliases rather than
  ambiguous ProcessId
• rename uses of pid as an expr name to childPid or sup

Ideally all internal bits of Supervisor would use phantom types to
make this a type level rather than name based distinction. This is a
first step towards that goal.

[tavisrudd]

Sorry, in my raising a newborn / sleep-deprived state I managed to miss a hunk on the first commit. You'll want to squash these commits together.

[tavisrudd]

@hyperthunk have you seen this SupervisorTest case fail ephemerally before? I can't replicate it locally.
https://travis-ci.org/haskell-distributed/distributed-process-platform/jobs/24319462#L1111

ExitShutdown Is Considered Normal (Closure): [Failed]

unexpected child ref: ("transient-worker",Just (ChildRunning pid://127.0.0.1:8080:0:141,ChildSpec {childKey = "transient-worker", childType = Worker, childRestart = Transient, childStop = TerminateImmediately, childStart = RunClosure (Closure (Static (StaticApply (StaticLabel "$const") (StaticLabel "Main.blockIndefinitely"))) ""), childRegName = Nothing}))

[hyperthunk]

@tavisrudd - I haven't seen that one for a while now, but IIRC I did see it previously. I'll have to dig into it.