fixes #206

This only affects classes that have getters and/or setters. I went with the solution here: #206 (comment)

One edge-case to note is this:

class Foo {
    prop = "foo";
}
class Bar extends Foo {
    get prop() { return "bar"; }
}
print((new Bar()).prop);

Because prop is assigned directly to the instance in the base class, the getter defined in the derived class would never be accessed. My solution to this is to detect this specific case and set the instance's property to back nil in derived class' constructor.

Bar.prototype.____constructor = function(self, ...)
    Foo.prototype.____constructor(self, ...);
    rawset(self, "prop", nil);
end;

Since the base property is effectively inaccessible in this situation, I don't think this should cause any undesirable side-effects.