uart: can now write more stuff

uart/uart.c

@@ -21,3 +21,9 @@ char uart_read(struct uart* uart) {
 
   return uart->data;
 }
+
+void uart_writen(struct uart* uart, const char* data, int len) {
+  for (int i = 0; i < len; ++i) {
+    uart_write(uart, data[i]);
+  }
+}