Add more assertions checking space and allocator mappings

[k-sareen]

In #1340, tests were failing because even though the mapping from semantics to allocator was correct, the mapping from space to allocator was not (since it accidentally passed a false to the create_space_mapping function). This was fixed in cd3bde3.

But the failure state was really confusing -- it was segfaulting in Rust code because the self pointer was nonsense. I think this sort of issue can be prevented by adding more assertions that check the mappings are correct. This is a fairly simple/innocuous mistake to make.