DIP18: Non-GC threads

[Peter Alexander]

I think this is an unnecessary addition.

I use D for game dev, and I avoid the GC + malloc like the 
plague, however I don't want to have to litter my code with 
distractions like nogc, @noheap or whatever keyword we choose.

It's easy enough to avoid these without expanding the type 
system. I add trace statements to the GC in druntime, and just 
avoid calling malloc. I think this is a more practical solution 
to this problem than extending the language and cluttering my 
code.