Proposal: Move `mapM` and `sequence` out of Traversable

Hi All, I have a proposal for the `base` library:  I would like to move `mapM` and `sequence` out of the class definition for `Traversable`, redefining them as toplevel aliases: ```haskell mapM :: Traversable t ⇒ Monad m ⇒ (a -> m b) -> t a -> m (t b) mapM = traverse sequence :: Traversable t ⇒ Monad m=> t (m a) -> m (t a) sequence = sequenceA ``` This slims `Traversable` by 50%. This would be a very small breaking change, which is completely tractable, but a great improvement for the ecosystem imo. Thoughts? What timeline should we shoot for with a change like this? Cheers, Emily