Expose alloc_slow. Add a section in user guide about allocation optimization

[qinsoon]

This PR exposes alloc_slow() to the bindings, adds a few public methods to allow bindings to implement allocation efficiently without duplicating mmtk-core code, and adds a section in the user guide to discuss allocation optimization.

The changes in this PR includes:

1. Expose alloc_slow() in memory_manager.
2. Add Mutator::allocator() to allow bindings to get a specific allocator from an allocator selector. Add Mutator::allocator_impl() to allow bindings to get a typed allocator from a selector.
3. Add Mutator::get_allocator_base_offset() to allow bindings to use a specific allocator without selector (for performance).
4. Add a section in the user guide about allocation optimization. Remove some unused SUMMARY.md in the user guide.
5. Add Address::as_mut_ref().
6. Expose the field for the fastpath bump pointer in some allocators.

Related discussion on Zulip: https://mmtk.zulipchat.com/#narrow/stream/262679-General/topic/Refilling.20BumpPointer.20using.20AllocatorInfo/near/394142997

[playXE]

This looks good to me. I wonder if alloc_slow could be enhanced to accept &mut BumpPointer as well? So binding does not have to manually update cursor and limit fields. But I guess this won't work if binding does not embed BumpPointer in its own TLS.

[wks]

Note: If the default_bump_pointer is initialized with (0, 0), it will be updated with meaningful values here in the first allocation.

[wks]

This looks good to me. I wonder if alloc_slow could be enhanced to accept &mut BumpPointer as well? So binding does not have to manually update cursor and limit fields. But I guess this won't work if binding does not embed BumpPointer in its own TLS.

That's right. VMs may store the cursor and limit in its own style, and may not have the same layout as our struct BumpPointer. I guess some VMs already has bump-pointer allocators in their own GC before adopting MMTk. The JIT compiler may even put the cursor and the limit in registers.

[wks]

LGTM