Add a warning flag that warns when a datatype could be a newtype

More often than not, newtype is desirable over data if a type has a single field. I propose a warning flag -Wdata-could-be-newtype that would warn users if a data type they've defined has a single field (and thus could be a newtype). It would need to be explicitly enabled (and of course enabled by -Weverything). An example (relevant Trac ticket: #15995 (closed)):

-- Control.Concurrent.QSem:

data QSem = QSem !(MVar (Int, [MVar ()], [MVar ()]))

could be a newtype

-- Control.Concurrent.QSem:

newtype QSem = QSem (MVar (Int, [MVar ()], [MVar ()]))