std.fmt.format: small optimization to avoid runtime bloat
std.fmt.format: small optimization to avoid runtime bloat

When there are no format parameters, it simply calls `writeAll`. This
has the effect of no longer emitting a compile error for using `{}` and
not having any parameters, however, at this point in the development
process of Zig I think that tradeoff is worthwhile.

On the other hand, it might be OK to simply define formatting to work
this way. It's a common pattern to use the formatting function's format
string `"like this", .{}` instead of `"{}", .{"like this"}`, which can
lead to accidentally putting control characters in the formatting
string, however, with this change that works just fine.

andrewrk/zig,andrewrk/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig