Use an actual Ruby instance variable in Pathname

* Fixes #1460
* So we are sure to be fully compatible with MRI
* #instance_variable_set must be reflected and redefining it on Pathname seems
  a very bad idea (even though Pathname is immutable without #instance_variable_set)
* We need anyway to add the instance variable for subclasses which access @path
  directly
* An alternative would be to define a YAML deserialization hook
  but we may face other compatibility issues