Merge SFT with Space

[qinsoon]

trait SFT (Space Function Table) was created so that we can use it as trait objects, as at that time, Space<VM, PR> could not be used as trait objects. To solve this, SFT was introduced and some of functions in Space were moved to SFT.

#108 removed type parameters of Space<VM>, and makes it possible to merge what is in SFT back to Space. We should think about whether we should merge SFT back with Space.

Pros:

• Less traits.
• One Space trait, no confusion.

Cons:

• As we plan to provide a more efficient mechanism to implement SFT (Per-Space "Dynamic Dispatch" #55), the merging will make it unclear what would be in space function table.

[wks]

A related type is SpaceDescriptor. Similar to SFT which maps object addresses to dynamically dispatched Space instances, a SpaceDescriptor maps object addresses to some selected space metadata. They actually serve similar purposes.

Related links:

• Two-step spaces instantiation #984 This issue mentioned the space indices in SpaceDescriptor and DenseChunkMap (an implementation of SFTMap).
• Remove descriptor_map #952 An attempt to remove SpaceDescriptor, but was cancelled because we still think SpaceDescriptor is useful.
• https://mmtk.zulipchat.com/#narrow/stream/262673-mmtk-core/topic/descriptor_map Further discussion.