Generate a $proxy class for all abstract classes
and use them when the return type is abstract,
similar to interface proxies.

This change also adds "abstract: true" to all
interface members, so the proxy generator can
treat interfaces and classes polymorphically.

Added a compliance test "returnAbstract" which
verifies this behavior.

Fixes #220
Related to #223 (.NET)
Related to aws/aws-cdk#680 (require jsii update)