Fix SI-3326.

The heart of the problem - we want to retain the ordering when
using `++` on sorted maps.

There are 2 `++` overloads - a generic one in traversables and
a map-specific one in `MapLike` - which knows about the ordering.

The problem here is that the expected return type for the expression
in which `++` appears drives the decision of the overload that needs
to be taken.
The `collection.SortedMap` does not have `++` overridden to return
`SortedMap`, but `immutable.Map` instead.
This is why `collection.SortedMap` used to resort to the generic
`TraversableLike.++` which knows nothing about the ordering.

To avoid `collection.SortedMap`s resort to the more generic `TraverableLike.++`,
we override the `MapLike.++` overload in `collection.SortedMap` to return
the proper type `SortedMap`.

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