Add support for method references invoking extension methods

[EotT123]

#664

[rsmckinney]

Looking good! Does it handle all types of method references?

Foo foo = getFoo();
// where
(a, b) -> foo.bar(a, b)
// replaces
foo::bar

[EotT123]

Yes, there is already a test for that case.
The following code

Map<String, Integer> map = new LinkedHashMap<>();
map.add( "Foo", 1 );
map.add( "Bar", 2 );
assertEquals( Arrays.asList( "Foo1", "Bar2"), 
    map.map( String::concatenate ).collect( Collectors.toList()));

Is replaced by:

Map<String, Integer> map = new LinkedHashMap();
MyMapExt.add(map, "Foo", 1);
MyMapExt.add(map, "Bar", 2);
assertEquals(Arrays.asList("Foo1", "Bar2"), 
    MyMapExt.map(map, (var0, var1) -> MyStringExt.concatenate(var0, var1)).collect(Collectors.toList()));

I've already implemented support for extension methods and structural interfaces.
The remaining tasks are handling methods that return auto and tuples. Since tuples are part of a separate module, I'll need to do some refactoring to make the code reusable across both modules. But I'm not sure if handling auto and tuples is possible?
Since tuples are located in another module, maybe this can be postponed to another PR?

[EotT123]

@rsmckinney I believe this PR is ready. I've tested it with a real project, and I didn't encounter any issues.

IntelliJ is still complaining ("Extension method ... must be invoked as a lambda expression here"), but the code is compiling fine. So I guess an update of the IntelliJ plugin is also needed.