GC again

[Frank Benoit (keinfarbton)]

> Garbage Collector: We now use Boehm GC in precise mode as opposed to fully 
> conservative mode. We also use it with a precise set of GC roots which 
> greatly improved Garbage Collection performance.

The explanation on wikipedia are a good starting point
http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29

> What does this mean and how hard would it be for D to do this as opposed to 
> doing a full generational GC?  Maybe we could do this as a short term 
> improvement while we wait for a generational GC in the long term.

At the moment the GC has no information, what values in memory are of
pointer type, and which one are not. So he takes them all as pointers.
This means conservative (vs. precise).

To do a precise GC, this information must be available and that will
need support in the compiler.

To be precise is a precondition for a generational GC.