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

[via Digitalmars-d-bugs]

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

--- Comment #2 from Ketmar Dark <ketmar at ketmar.no-ip.org> ---
i just thought that `.clear!true` is better. i don't know why: probably 'cause
i just love D templates and want to use 'em everywhere. ;-)

still, it's a matter of simple renaming, i don't have real API preferences
here. something like `.clear!(init:true)` may be better, but D doesn't support
that. T_T

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