compiler support added for precise GC

[Walter Bright]

Just checked it in. Of course, it doesn't actually do precise GC, it is just 
thrown over the wall for the library devs who are itching to get started on it.

I added a getGCInfo() method to TypeInfo that returns an immutable(void)*. This 
pointer can be anything - a pointer to data, to code, whatever, that implements 
whatever the GC might need to do precise collections. The value is generated by 
the template GCInfo(T) in object.d.

Some observations:

1. if there are no pointers in the allocated data, the GCInfo(T) should be null. 
This enables a fast static check with no indirection for this most common case.

2. closure memory is allocated by calling _d_allocmemory. For now, it should 
just use the old conservative mark/sweep. Later, I can add a GCInfo(T) for it.

3. Many types will follow similar patterns:

    ptr .. int .. ptr .. int

    ptr .. ptr

    int .. ptr

I suggest that specializations exist for these to avoid generating innumerable 
identical data structures or functions. In fact, if they are named with names like:
    scanpipi()
    scanpp()
    scanip()

then the linker will automatically remove duplicates.

4. Stack scanning remains imprecise, and should use the usual conservative method.

5. The "has pointers" bit array can, of course, be eliminated.

6. I suggest the GCInfo pointer be stored at the end of the allocated block, as 
then it won't affect the alignment of the allocated data.

Release the hounds!