Precise GC state

[Temtaime]

Hi all !
https://github.com/dlang/druntime/pull/1603

Can someone investigate and bring it to us ?
4 years passed from gsoc 2013 and there's still no gc.
Many apps suffers from false pointers and bringing such a gc will 
help those who affected by it.
It seems all the tests are passed except win32 because of optlink 
failures.
Maybe there's some chance to accelerate this PR ?

Thanks all