`-> Solo a` functions for lazy maps

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

```hs
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.

Start by locating the lazy-map APIs and comparing the existing insertWith and unionWith signatures with the proposed Solo-based forms in the issue. Determine the complete set of related functions to cover and define done as a consistent API addition with appropriate tests for thunk selection and behavior.

Written by the indexing model from the issue text.