Multiple alias this redux

[deadalnix via Digitalmars-d]

Anything, but please make this happen. Considering how alias this 
is implemented in the first place, I really doubt a simple 
implementation exists. Maybe a simpler one, I'm not familiar 
enough with the codebase to judge, but certainly not a simple one 
(dug into the alias this code not so long ago).

Considering this, I do think this is reasonable to accept some 
level a complexity in the PR that may not seems strictly required 
at first glance.

alias this, is a complex feature as it interact with identifier 
resolution, casting, choosing overload when calling and some 
more. This is the nature of the beast.