[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 #6 from Steven Schveighoffer <schveiguy at yahoo.com> ---
Thanks, this looks really good. I like the returning of itself, so you just put
ensureAllocated on the call and it just works in all cases where you normally
have an AA.

When I get a chance, I'll make a PR. Thanks for the tests too!

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