Eo: Implement eo_override() to enable overriding functions of objects.

Eo: Implement eo_override() to enable overriding functions of objects.

This change lets you override the functions of objects so that those
functions will be called instead of the functions of the class. This
lets you change objects on the fly and makes using the delegate pattern
easier (no need to create a class every time anymore).
You can see the newly added tests (in this commit) for usage examples.

@feature