compile-time checked format strings

[kdevel]

occasion: 
http://forum.dlang.org/thread/mutegviphsjwqzqfouhs@forum.dlang.org?page=3#post-mailman.2136.1515709204.9493.digitalmars-d-announce:40puremagic.com

dmd checks the types but does not count the arguments.

ctcfs.d
```
import std.stdio;
import std.math;

void unit (T) ()
{
    auto pi = 4 * atan (T (1));
    writefln!"%30.24f" (pi, pi); // no error!
    writefln!"%30.24f %30.24f" (pi, pi); // OK
//   writefln!"%30.24d %30.24f" (pi, pi); // OK:  "incompatible 
format character
    writefln!"%30.24f %30.24f %30.24f" (pi, pi); // no error
}

void main ()
{
    unit!float;
}
```

$ dmd ctcfs.d
$ ./ctcfs
     3.141592741012573242187500
     3.141592741012573242187500     3.141592741012573242187500
     3.141592741012573242187500     3.141592741012573242187500

std.format.FormatException@/.../dmd2/linux/bin64/../../src/phobos/std/format.d(479): Orphan format specifier: %f
----------------
/.../dmd2/linux/bin64/../../src/phobos/std/exception.d:615 pure 
@safe bool 
std.exception.enforceEx!(std.format.FormatException).enforceEx!(bool).enforceEx(bool, lazy immutable(char)[], immutable(char)[], ulong) [0x44503c]
/.../dmd2/linux/bin64/../../src/phobos/std/format.d:479 @safe 
uint std.format.formattedWrite!(std.stdio.File.LockingTextWriter, 
char, float, float).formattedWrite(ref 
std.stdio.File.LockingTextWriter, const(char[]), float, float) 
[0x44c436]
/.../dmd2/linux/bin64/../../src/phobos/std/stdio.d:1496 @safe 
void std.stdio.File.writefln!(char, float, 
float).writefln(const(char[]), float, float) [0x44c340]
/.../dmd2/linux/bin64/../../src/phobos/std/stdio.d:3797 @safe 
void std.stdio.writefln!(char, float, 
float).writefln(const(char[]), float, float) [0x44c2b7]
/.../dmd2/linux/bin64/../../src/phobos/std/stdio.d:3791 @safe 
void std.stdio.writefln!("%30.24f %30.24f %30.24f", float, 
float).writefln(float, float) [0x44da27]
ctcfs.d:10 @safe void ctcfs.unit!(float).unit() [0x443673]
ctcfs.d:15 _Dmain [0x443614]