This commit adds the System.Process.CommunicationHandle module, which
provides the cross-platform CommunicationHandle abstraction which allows
Handles to be passed to child processes for inter-process communication.

A high-level API is provided by the function
`readCreateProcessWithExitCodeCommunicationHandle`, which can be
consulted for further details about how the functionality is meant to be
used.

To test this functionality, we created a new "cli-child" executable
component to the process-tests package. To work around Cabal bug #9854,
it was necessary to change the build-type of the package to `Custom`, in
order to make the "cli-child" executable visible when running the test-suite.
The custom Setup.hs script contains more details about the problem.