ImmixAllocator should use BumpAllocator fastpath

[qinsoon]

ImmixAllocator uses the same fast path allocation as BumpAllocator. Also a VM's compiler can generate immix fastpath in the same way as bump pointer allocator (except that the field offsets may be different).

It is unclear how to achieve this in the code. One possible implementation is to embed a BumpAllocator in ImmixAllocator, and use it for the fastpath allocation.

A JIT compiler would only need to know what allocator fastpath to generate. So maybe we can have a separate type for AllocatorFastpath?