ivar-simple: Write once concurrency primitives.

What is this?
-------------

ivar-simple provides immutable, write-once variables (IVars) in the
Data.IVar.Simple module.

It also provides two more experimental channel implementations built on
top of IVars,

    Data.IVar.Simple.IChan:
        multi-cast channels with write-once semantics.

    Data.IVar.Simple.MIChan:
        channels with write semantics similar to Control.Concurrent.Chan

Comparison to data-ivar
-----------------------

Both data-ivar and ivar-simple provide a write-once variable. That's where
the similarities end:

- Reading an IVar with data-ivar is an IO operation. In ivar-simple it's a
  pure function.
- data-ivar provides a 'Reader' monoid, monad, etc. for reading from one of
  several IVars. ivar-simple has no such functionality.
- The data-ivar implementation can, in principle, add arbitrary IO actions
  that are called when an IVar is written. (This detail is not exposed,
  however)
- Technically, ivar-simple tries for efficiency by exploiting the existing
  locking structures in the RTS; in particular, reading a full IVar for a
  second time is as cheap as evaluating a record selector.
  I have no performance numbers for this.