Object::len() and Object::is_empty() conflict with inherent len(), is_empty()

[SergioBenitez]

Calling self.len() inside of any method implementation (in particular, enumerate()) calls the trait method, which is unlikely to be what's desired. The only way to prevent this is to rename len() and is_empty() to something else.

I would suggest length() or enumerator_len() instead of len(). For is_empty(), maybe has_no_elements() or is_void() or is_vacant()?

[mitsuhiko]

Yeah that's surprisingly annoying. In the rest of the engine is_empty is just the inverse (is_true). I just never really liked that name because it's not literally true, but true as far as the conditionals is concerned, but we could use this here too. Then at least it's consistent. I think enumerator_len is reasonable because you're supposed to match them anyways and that way this is more explicit.