format string as described in std.format.formattedWrite
arguments to be formatted
OutBuffer b = new OutBuffer(); b.writef("a%sb", 16); assert(b.toString() == "a16b");
OutBuffer b = new OutBuffer(); b.writef!"a%sb"(16); assert(b.toString() == "a16b");
Formats and writes its arguments in text format to the OutBuffer.