Dense storage of elements of D's builtin associative arrays (AAs)

[Per Nordlöw]

Would it be relevant to change D's AA implementation to use dense 
storage of its elements instead of to individual bucket storage?

I'm guessing not because

1. pointer returned by `in`-operator
2. range returned by `byKey`, `byValue`, and `byKeyValue`?

may indefinitely outlive the current store upon grow-reallocation 
potentially causing massively fragmented memory of previous 
stores.