Dict: Add divideAndConquer helper.

[showell]

This function will allow folks to do O(logN)
traversals in some situations where the opaque
nature of Dict currently would prevent those
optimizations.

An example use case would be that you only want
to render the visible portion of a line chart
whose values were stored in a Dict.

As I mention in the docs, it's for advanced users.

Example usage:

example =
    [ ( 5, "fred" )
    , ( 6, "joe" )
    , ( 7, "mary" )
    , ( 8, "ann" )
    , ( 9, "bob" )
    , ( 10, "carl" )
    ]
        |> Dict.fromList

_ =
    Debug.log "hello" (Dict.divideAndConquer example)

yo =
    Debug.log "hello" (Dict.divideAndConquer Dict.empty)