Add dstringwriter.

test/unit/stdlib/io/dstringstream.c3

@@ -0,0 +1,17 @@
+module std::io @test;
+
+fn void! test_writing()
+{
+	DString foo;
+	foo.init();
+	Stream s = foo.as_stream();
+	s.write("hello")!!;
+	s.write_byte('-')!!;
+	s.write("what?")!!;
+	ByteReader r;
+	String test_str = "2134";
+	r.init(test_str);
+	s.read_from(&&r.as_stream())?;
+	String o = foo.str();
+	assert(o == "hello-what?2134");
+}