Decouple Node and RubyObject instances.

The classes Node, RubyObject and RubyMethod are now decoupled from each other.
Although the latter two still provide the means to create a definition based on
a Node instance (using RubyObject.new_from_node) they no longer rely on Node
instances for generic use.

Signed-off-by: Yorick Peterse <yorickpeterse@gmail.com>