[interop] Array-like object do not implement length and other Array methods

[fniephaus]

According to the polyglot docs, foreign objects understand size, which is an alias for length. In fact, foreign arrays should appear as Ruby Arrays and hence should also understand all other messages of Array. That would significantly improve portability of foreign code and it would be in line with what @chumer told me.

[eregon]

Agreed, this would make sense but will probably be quite some effort to make it work, especially considering Array.public_instance_methods(false).size # => 104 and quite a few of these currently work on the internal Array representation directly.
Maybe we need a separate implementation of these methods for foreign arrays, which only support the array elements interop.

[eregon]

This is done now (#2149 (comment)).