This lets dejafu test CoCo-style properties, and introduces a
dependency on [leancheck][].
For example, consider this statement about MVars: "using readMVar is
better than a takeMVar followed by a putMVar because the former is
atomic but the latter is not."
This module can test properties like that:
```
sig e = Sig
{ initialise = maybe newEmptyMVar newMVar
, observe = \v _ -> tryReadMVar v
, interfere = \v s -> tryTakeMVar v >>
maybe (pure ()) (void . tryPutMVar v) s
, expr = e
}
> check $ sig (void . readMVar) `equivalentTo`
sig (\v -> takeMVar v >>= putMVar v)
*** Failure: (seed Just ())
left: [(Nothing,Just ())]
right: [(Nothing,Just ()),(Just Deadlock,Just ())]
```
The two expressions are not equivalent, and we get given the
counterexample!
In addition to "equivalentTo", there is also "refines" and
"strictlyRefines" (and negated variants)
[leancheck]: https://hackage.haskell.org/package/leancheck
The new 'concurrency' package is starting at version 1.0.0.0 because the
API is already very mature (copied from base).
This breaks the dejafu-0.2 compatibility of async-dejafu.
Closes#51.
