Fix unordered keyed list mapping

Due to legacy reasons, the following snippet:

container foo {
    list bar {
        key baz;
        leaf baz {
            type string;
        }
    }
}

ends up being mapped as this:

interface Foo {
    List<Bar> getBar();
}

which ends up being nasty for multiple reasons:

1. default order-by is "system", yet we are married to order-dependent List.equals(), yet
2. the BI layer will interpret this as a freely-reorderable Map
3. there is no provision to look up elements by their key and hence users must perform linear searches

The mapping for keyed lists with "order-by system", the mapping should result in the following:

interface Foo {
    Map<BarKey, Bar> getBar();
}

which properly captures the contract here, binding use to Map.equals(). Note that unkeyed lists must not be affected, nor should the "order-by user" lists be changed.

For dealing with user-ordered lists we will need a custom interface, which is an extension of List and provides a secondary map-like lookups, still deriving equality from List.equals().