compile-time checked format strings

[kdevel]

On Saturday, 13 January 2018 at 19:40:09 UTC, Adam D. Ruppe wrote:
> For ints, it catches all that, but for float, it just bails out 
> of the check as soon as it actually *succeeds* - because that 
> kills CTFE.

Confirmed. Thanks!

args.d
```
import std.stdio;

void main ()
{
//   writefln!"%2.2d %2.2d" (1); // OK: Orphan format specifier
//    writefln!"%2.2d" (1, 2); // OK: Orphan format arguments:
}
```