A variation of "Control.Concurrent.Chan" from base, which allows to the easy
creation of broadcast channels without the space-leaks that may arise from
using 'Control.Concurrent.Chan.dupChan'. The 'Control.Concurrent.Chan.Chan'
type from "Control.Concurrent.Chan" consists of both a read and write end.
This presents a problem when one wants to have a broadcast channel that, at
times, has zero listeners. To write to a 'Control.Concurrent.Chan.Chan' there
must always be a read end and this read end will hold ALL messages alive until
read. The simple solution applied in this module is to separate read and write
ends. As a result, any messages written to the write end can be immediately
garbage collected if there are no active read ends, avoding space leaks.