Remove unsafety for uninitialized allocatr. Initialize with unusable space instead.

[qinsoon]

Inspired by #1483.

• Initialize all the allocators with unusable space first.
• Then initialize the allocators that a plan may use with proper spaces.
• Refactor allocators so all allocators reference a &dyn Space. This requires changing bindings, as the layout of allocators are changed.

[qinsoon]

I will make binding side changes if we think this is a good idea.

[k-sareen]

I think this solution is preferred over #1483 since we need to ensure that MutatorContext is FFI-safe in case bindings want to embed it entirely.

[wks]

I think this solution is preferred over #1483 since we need to ensure that MutatorContext is FFI-safe in case bindings want to embed it entirely.

Alternatively, we can first investigate whether disallowing embedding Mutator in favor for embedding only BumpPointer can keep the performance good in OpenJDK. If that's possible, we can refactor the Mutator even more aggressively.

See:

• Remove repr(C) from Mutator #1318
• This comment mentioned OpenJDK performance. Remove repr(C) from Mutator #1318 (comment)

[qinsoon]

#1483 seems good to me. If it is merged, we can close this PR.