[Issue 16269] add `aa.ensureAllocated` method to associative array to clear and initialize it

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16269

--- Comment #7 from Steven Schveighoffer <schveiguy at yahoo.com> ---
Just adding a note for myself when I make the PR, the AA is and always will be
an implementation pointer as the only member. I will check for null pointer in
wrapper rather than the runtime code to avoid extra opaque calls.

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