Implementing multiple inheritance in D

[kenji hara]

Thanks for your comment.

> Could you please provide more detail on what you fixed and what the
> resulting context is?

I posted 'Issue 5973 - alias this is not considered with superclass
lookup' for dmd issue.
http://d.puremagic.com/issues/show_bug.cgi?id=5973
The issue is that dmd does not consider alias this in super classes on
member lookup.

More clean code: https://github.com/9rnsr/scrap/blob/master/multiInherit.d
New feature:
- Support class that have explicit constructor.
  See X.this code with -version=ExplicitCtor.

May not support:
- Diamond inheritance

Kenji Hara