core/function: bug 1729 was fixed in PyPy 2.3, so moving deepcopy() to a proper location
travis.yml: added a note about PyPy
core/walker: automatically replace an empty statement block with a single `pass`
TODO: removed an item
tests/core/walker: added a test for skip_fields/walk_field controls
tests/core/walker: separated skip_fields() and walk_field() tests
core/walker: visiting fields only if the node was not changed
tests/core/walker: test the logic of walking fields of changed and unchanged nodes
core/walker: added a docstring