lib/utils/stdout_helpers: Make mp_hal_stdout_tx_strn_cooked efficient.

To reduce the number of calls to mp_hal_stdout_tx_strn and improve the
overall throughput of printing data.  This implementation is taken from
ports/stm32/mphalport.c.
Signed-off-by: Damien George <damien@micropython.org>

lib/utils/stdout_helpers.c

+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Damien P. George
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
 #include <string.h>
-#include <unistd.h>
-#include "py/mpconfig.h"
 #include "py/mphal.h"
 
 /*
@@ -10,13 +34,25 @@
  */
 
 // Send "cooked" string of given length, where every occurrence of
-// LF character is replaced with CR LF.
+// LF character is replaced with CR LF ("\n" is converted to "\r\n").
+// This is an optimised version to reduce the number of calls made
+// to mp_hal_stdout_tx_strn.
 void mp_hal_stdout_tx_strn_cooked(const char *str, size_t len) {
+    const char *last = str;
     while (len--) {
         if (*str == '\n') {
-            mp_hal_stdout_tx_strn("\r", 1);
+            if (str > last) {
+                mp_hal_stdout_tx_strn(last, str - last);
+            }
+            mp_hal_stdout_tx_strn("\r\n", 2);
+            ++str;
+            last = str;
+        } else {
+            ++str;
         }
-        mp_hal_stdout_tx_strn(str++, 1);
+    }
+    if (str > last) {
+        mp_hal_stdout_tx_strn(last, str - last);
     }
 }