Distinct TVarIds for various TVar roles

[coot]

TVars are used to emulate TMVars and MVars and a few other shared variables, and thus can have
many different roles. For each role TVarId provides a constructor,
which makes it easier to distinguish them in a trace.

• TVar
• TMVar
• MVar
• TQueue
• TBQueue
• TSem
• TChan - left as a todo.

ex0 :: IOSim s ()
ex0 = do
    atomically $ newTVar ""
    return ()

ex1 :: IOSim s ()
ex1 = do
    atomically $ newTMVar ""
    return ()

ex2 :: IOSim s ()
ex2 = do
    newEmptyMVar
    return ()

λ putStrLn  $ ppTrace $ runSimTrace ex0
0s - Thread []   main - TxCommitted [] [TVarId 0]
0s - Thread []   main - Unblocked []
0s - Thread []   main - Deschedule Yield
0s - Thread []   main - ThreadFinished
0s - Thread []   main - MainReturn () []
λ putStrLn  $ ppTrace $ runSimTrace ex1
0s - Thread []   main - TxCommitted [] [TMVarId 0]
0s - Thread []   main - Unblocked []
0s - Thread []   main - Deschedule Yield
0s - Thread []   main - ThreadFinished
0s - Thread []   main - MainReturn () []
λ putStrLn  $ ppTrace $ runSimTrace ex2
0s - Thread []   main - TxCommitted [] [MVarId 0]
0s - Thread []   main - Unblocked []
0s - Thread []   main - Deschedule Yield
0s - Thread []   main - ThreadFinished
0s - Thread []   main - MainReturn () []

[jasagredo]

Looks good!

[coot]

I pushed some stylistic & haddock changes.