Ch7 Align definitions of combineList and sequence

[milesfrain]

This chapter describes how to create combineList, then compares it to the more general sequence:

combineList :: forall f a. Applicative f => List (f a) -> f (List a)
sequence    :: forall a m. Applicative m => t    (m a) -> m (   t a)

The connection would be more obvious though if the same type variables were used:

combineList :: forall a m. Applicative m => List (m a) -> m (List a)
sequence    :: forall a m. Applicative m => t    (m a) -> m (   t a)

It's tempting to just replace f with m in combineList but then there's a disconnect with an earlier connection to Apply which uses f.

class Apply f <= Applicative f where
  pure :: forall a. a -> f a

So this seems like a bigger task of settling on a naming convention to use for type variables and then updating the libraries.