This patch makes all HeapBlock allocations aligned to their block size,
enabling us to find the HeapBlock* for a given Cell* by simply masking
bits off of the cell address.
Use this to make a simple Heap& getter for Cell, which lets us avoid
plumbing the Heap& everywhere.
Don't visit cells that are already marked. This prevents the marking
phase from looping forever when two cells refer to each other.
Also do the marking directly from the CellVisitor, removing another
unnecessary phase of the collector. :^)
Objects can now be allocated via the interpreter's heap. Objects that
are allocated in this way will need to be provably reachable from at
least one of the known object graph roots.
The roots are currently determined by Heap::collect_roots().
Anything that wants be collectable garbage should inherit from Cell,
the fundamental atom of the GC heap.
This is pretty neat! :^)
