Add new reference phase, delete leafref attributes.

[robshakir]

 * (M) pyang/statements.py
   - Add a validation phase where changes to types with references
     by deviations can be made ahead of validating any reference.
   - Remove the `i_leafref` attribute when a deviation changes the
     type of a leaf to a type that isn't a leafref.
 * (A) test/test_issues/test_i771
   - Add a test case validating the fix to issue #771.

Fixes #771.