object extension, overriding

[graydon]

Make anonymous objects that delegate to base objects -- adding or redefining methods along the way -- work.

[lkuper]

Now that simple anonymous objects are working, to prevent confusion,
I'm breaking down the rest of the work I'm doing with anonymous
objects into four separate bugs:

• Issue Adding support for extending objects with new fields #538: Adding support for extending objects with new fields.
  This will be a pretty big feature, involving the parser and,
  importantly, the resolver, as well as the typechecking and
  translation passes. Currently xfail'd test:
  src/test/run-pass/anon-objs-with-fields.rs.
• Issue Adding support for 'degenerate' anonymous objects #539: Addding support for 'degenerate' anonymous objects
  that don't add any new methods or fields. This might seem
  trivial, but part of making it work is ensuring that calls to
  anonymous objects "fall through" appropriately, so that calls to
  methods that were on the original object still work when made on
  the anonymous object. Currently xfail'd test:
  src/test/run-pass/anon-obj-degenerate.rs.
• Issue Adding support for extending objects with methods that contain simple self-calls #540: Adding support for extending objects with methods that
  contain simple self-calls (no overloading). I think the place
  this is currently failing is in typechecking, although I need to
  make sure. Currently xfail'd test:
  src/test/run-pass/anon-obj-with-self-call.rs.
• Issue Adding support for extending objects with methods that override existing methods in the presence of self-calls #543: Adding support for extending objects with methods that
  overload existing methods in the presence of self-calls. This is
  tricky because we have to make sure that the meaning of 'self' is
  also overloaded. Currently xfail'd test:
  src/test/run-pass/anon-obj-overloading.rs.

As far as I can tell, those four issues supersede this one, so I'm
closing this one and will continue working on the others. Let me know
if you see something I missed.