Why is the identifier "printf" known by the frontend?

[Walter Bright]

Wolfgang Draxinger wrote:
> What is the reason for that? "import std.stdio" is not a real
> difficult thing to type.

Because I find it verrryy handy in writing minimal test cases. writefln, 
in order to work, requires much of D and Phobos to be working properly. 
printf, being part of the C library, doesn't require any of Phobos and 
very little of D to work, so it's handy when things are broken.