Added reminder note in Process to remind myself why I used an MVar in runWorker.
Now sending messages to Handles is thread-safe in Process.

Before I added a lock there was the theoretical
possibility that a context-switch could happen
in the middle of sending a message.
Cosmetic:  Got rid of the type alias constraint in forkWorkerThread.
Now Process.runWorker is visitor-mode aware.
launchVisitorStartingFrom --> runVisitor
Added a note to the package docs about hslogger.
Dropped Mode from the end of MessageForSupervisorForMode.

This was done because it was the only type that
had "Mode" on the end that constructed a type
from a visitor mode so it kept sticking out like a
sore thumb.