Fix a really insidious bug in 'SysfsMockT' impl.
Fix a really insidious bug in 'SysfsMockT' impl.

'SysfsMockT's instance of 'MonadSysfs' had a 'MonadSysfs' constraint:

instance (MonadSysfs m, MonadThrow m) => M.MonadSysfs (SysfsMockT m) where
  ...

This caused all kinds of trouble when trying to use 'SysfsGpioMock' (or
similar stacked transformers) because GHC kept complaining that there
was no instance of 'MonadSysfs Identity' (or similar).

This was a really tough bug to find and I wish GHC had warned me that I
had made a nonsense instance of 'MonadSysfs'!

dhess/gpio