[containers] Proposal: Change to the Data.Map Monoid

[Nick Partridge]

Hi,

Currently the Monoid instance for Data.Map is implemented using
union/unions, which are left biased. On key collision, it discards values
from the right hand side of `mappend` -
https://github.com/ghc/packages-containers/blob/bae098fb0a3994bc2b0ec3313004b40cd097ed8d/Data/Map/Base.hs#L341-L344

If you compare this with the Monoid for Maybe, it's like we're defaulting
to First as the monoid instance for maps.

A more useful instance, however very much a breaking change, would be to
make the instance depend on a Monoid (or better yet, a Semigroup) for the
values in the map:

instance Monoid v => Monoid (Map k v) where
    mappend = unionWith mappend

This lets us build up maps with values in a useful Monoid, and mappend them
with gusto.

Thoughts?

- Nick Partridge

Discussion period: 2 weeks.
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