First Draft: Making printf @safe

[Walter Bright]

I compiled:
```D
import std.stdio;

void main()
{
     double d;
     writef("value = %s\n", d);
}
```
which produces an object file that is 444,272 bytes in size.
```D
import core.stdc.stdio;

void main()
{
     double d;
     printff("value = %g\n", d);
}
```
and the object file is 4,092 bytes in size.