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.

lib/std/fmt.zig

@@ -88,6 +88,8 @@ pub fn format(
     if (args.len > ArgSetType.bit_count) {
         @compileError("32 arguments max are supported per format call");
     }
+    if (args.len == 0)
+        return writer.writeAll(fmt);
 
     const State = enum {
         Start,