ExpressionTransformer: fix for Correspondence.item->get in language module
ClassTransformer #991: Always generate impl accessor methods even when there's no inner class

because we need to be able to access them from interface methods that refer to their super type methods
ExpressionTransformer #991: when in a companion class, use the interface impl accessor method to invoke super methods
Test for #991: Invoking super interface methods in interfaces
ExpressionTransformer #991: new supertype for comprehension iterators is AbstractIterator

So that we don't have to generate mixin supertype accessors
ClassTransformer #991: tweaked return type of mixin supertype accessors
Updated test for #991: mixins need supertype accessor methods
Updated tests .src for #991: mixins need supertype accessor methods, and comprehension iterators must extend AbstractIterator
BoxingVisitor: change for Correspondence.item->get
ExpressionTransformer: respect erasure flags for item access
Updated test .src and .ceylon for Correspondence.item->get rename