Make sure we see deeper inheritance than 1 level
Add method cachedWithOverrides which caches symbols and then uses the
cached symbol while overriding

This is a dirty quick fix for now until we rewrite the FunctionClosure
phase.

Added a correctness property, which seems to have a longish counterexample, found by fairz3. Is there a simpler counterexample?

Make signature of IsInstanceOf consistent with other Expr's
Add isInstOf Constructor
Correctly handle This in instantiateType
Correctly handle This in MethodLifting when it belongs to a subtype
An extra unit test

This reverts commit c4bf3e88.

Follow-up to 3e4281d8. Uses mutable state to imitate similar other
properties of ClassDefs.

'this' can be of a subtype while MethodLifting.
MethodLifting sometimes needs to insert type casts.