Provide default implementations for various MonadSTM methods

The `MonadSTM` module provides `newTMVarDefault` and many other default
implementations of `MonadSTM` methods.   They ought to be provided directly by `MonadSTM` class, e.g.

```hs
class MonadSTM m where
  ...
  default newTMVar :: (TMVar m a ~ TMVarDefault m a)
                   => a -> TMVar m a -> STM m a
  newTMVar = newTMVarDefault
```
