[Issue 12732] Add an Appender-like template that recursively builds a structure of Appender fields

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12732

--- Comment #1 from Andrej Mitrovic <andrej.mitrovich at gmail.com> ---
Oops, I failed to properly test that implementation. Lemme fix that up real
soon.

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