[Issue 13568] Support compile-time format strings in std.format

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=13568

--- Comment #11 from b2.temp at gmx.com ---
There's https://issues.dlang.org/show_bug.cgi?id=17381 that could be handled
the day std.format will be refact.

The strategy used to check statically the specifier (call format() and look if
there's been an exception) makes it impossible to fix.

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