This is a “monotonic
FxHashMap that, when shared, can be pushed to but not
otherwise mutated. We also box items in the map. This means we can safely provide
shared references into existing items in the
FxHashMap, because they will not be dropped
(from being removed) or moved (because they are boxed).
The API is completely tailored to what
memory.rs needs. It is still in
a separate file to minimize the amount of code that has to care about the unsafety.