[Issue 24320] Change std.writefln() into a regular variadic function not a template

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24320

--- Comment #1 from Walter Bright <bugzilla at digitalmars.com> ---
Arguments like `struct S { int x; int y; }` that have no corresponding format
specifier could use `%s` as the format specifier, and then the argument can be
wrapped in a template that provides a default toString() method.

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