Fix for #1908: be more strict in detecting variadicity of Callable in the presence of funky types
Test for #1908: UnsupportedOperationException when abstracting over no-args function
Updated test .src for #1908: UnsupportedOperationException when abstracting over no-args function

The update is weird, especially for #1533, but I don't think it's incorrect.
Updated tests due to typechecker changes