Fix MethodLifting and improvements
Fix MethodLifting and improvements

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

epfl-lara/stainless,epfl-lara/stainless,epfl-lara/stainless,epfl-lara/stainless