Implement standard traits for pointers and bump version to 0.2.0

[powergee]

• Resolves #1
  • For all pointer types, implemented Debug and Pointer.
  • For dereferenceable types (i.e., Rc and Snapshot), implemented PartialOrd, Ord, PartialEq, Eq, and Hash.
  • Implemented From by referring to the type relation graph.
• Resolves #2
  • Now Rc::eq and Snapshot::eq compare by objects they are pointing to. To compare by pointer values, use the ptr_eq method instead.
  • This patch is a breaking-change because it changes the behavior of a pre-existing API. Therefore, we need to increment the non-zero left-most version number (i.e., 0.1.0 -> 0.2.0).