Remove Array subclassing

Inheriting from Array (and other core types) is problematic:

  - It exposes a very wide interface with many methods that are not
    really relevant to the subclass.
  - It can cause some weird side effects, as many Array operations are
    in C and have hardcoded return values; for example, combining two
    array subclasses returns a new Array instead of the subclass.

Avoid these problems using delegation and the Enumerable module where
applicable.