std.fmt.format: tuple parameter instead of var args

[andrewrk]

Related: #208

Note that this is unrelated to C var args compatibility.

[giann]

Too bad this requires an empty struct when no parameters.

[andrewrk]

On the contrary, that is a feature! Yes, it takes a few more keystrokes when writing code. It does not feel as clean. However, when reading code, there is a 1:1 mapping between every callsite parameter and every function definition parameter.

When reading code that does not take advantage of format arguments, it is obvious how to edit the code to add format parameters. And, most importantly, an entire language feature can be deleted. There is no wondering how var args works, because it does not exist.

Block labels being required to return values from blocks is similarly unpopular, yet it teaches how to use block labels. The empty argument to formatted printing teaches how to use tuples.