Please be more careful when changing the language behavior

[Walter Bright]

All I can say is you're right, and we handled this badly.

alias this has a lot of problems, mainly because it tries to implement multiple 
inheritance without thinking it through. I've looked at fixing them, and 
essentially it is unfixable. We'll just have to live with the way it works, and 
shouldn't try to modify its behavior again.

Changes to the compiler rely very heavily on the test suite. If cases aren't in 
the test suite, we don't know when break things.