Precise garbage collection

[ollef]

See if we can get the LLVM gcroots stuff to work. Something that looks like a potential problem is that there's currently no support for marking non-pointer (e.g. structy) data as possibly containing pointers, so we may have to resort to conservative stack scanning or something and look into mostly copying GC.

With a GC that scans for pointers we might decide to use pointer tagging to decide whether something is a pointer or not (and reserve a bit in ints as well). If that's the case we'll need a new type for pointers to byte arrays so that their data is not scanned for pointers.