fmt! does not depend on commas between arguments

[Kimundi]

This code

io::println(fmt!("String %s number %d" "foo" 13));

compiles and produces the same output as

io::println(fmt!("String %s number %d", "foo", 13));

[catamorphism]

Not critical for 0.7. Nominating for milestone 1 (well-defined).

[graydon]

Accepted. The input to the fmt! macro needs to be written down.