Rename undefined to object

[71104]

"Everything is an object".

This one deserves release 0.10.0.

[71104]

Implement #84 before this one. Removal of LazyValue might actually mean there is no need of an ObjectValue class at all, as AbstractValue would become the root of all values.

[71104]

This naming is actually sound: given that everything is an object, some objects are classified as booleans, numbers, strings, lists, or lambdas, while others are not classified as any of the previous - they are undefined.

The naming convention will be kept along with the undefined keyword.

[71104]

Also, ObjectValue or UndefinedValue will not be removed in favor of rooting everything to AbstractValue because, unfortunately, we still have to support JavaScript by providing something that marshals to JavaScript undefined or JavaScript null, so those two special values will extend AbstractValue but not UndefinedValue. #94 has been opened to address this.