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.
