`-> Solo a` functions for lazy maps

[meooow25]

I don't know if it comes up in practice, but for lazy maps we can have functions like

insertWith :: Ord k => (a -> a -> Solo a) -> k -> a -> Map k a -> Map k a
unionWith :: Ord k => (a -> a -> Solo a) -> Map k a -> Map k a -> Map k a
...

This gives the caller precise control over which thunk to put in the map.
Right now, with (a -> a -> a) versions, it's always going to be the function application (unless the map operation inlines and simplifies).

I'm not planning to add these right away, but if someone wants them it makes a lot of sense to add them.