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

[Emily Pillmore]

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
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