How to synchronously shutdown the event manager loop

[Bas van Dijk]

Hello,

In my (still unreleased) usb-1.0 (https://github.com/basvandijk/usb)
library I use the GHC event manager for managing events from the
underlying `libusb` C library.

To work with the library a user has to initialize it using:

newCtx ∷ IO Ctx

The `Ctx` then allows the user to see the USB devices attached to the
system using:

getDevices ∷ Ctx → IO [Device]