Basic implementation of (s)printf using @(s)printf macros.

[dcarrera]

This is an implementation of printf() and sprintf() that relies on generated functions, and the existing @printf and @sprintf macros. The key code was posted on the Julia mailing list by Tim Holy. I just added the small wrappers to make printf() and sprintf().

[tonyhffong]

could you post some performance benchmark?

[lindahua]

I think eventually we should use Grisu algorithm.

[tonyhffong]

From the way it is implemented, I'm not seeing what it offers materially either from a performance or expressiveness perspective. I recommend we reject this PR.