Small String Optimization (esp8266#5690)

Reduce String memory overhead from 24 bytes to 16 bytes by limiting the
maximum string length to <64Kbytes (which is larger than heap so no
effective problem).

Add Small String Optimization, SSO, which instead of allocating pointers
to small strings on the heap will store the string in place of the
pointer in the class.  This should reduce memory fragmentation as

Save up to 12 chars (11 + \0) in String itself by using the terminating
\0 in the inline string as a flag to identify if this is a SSO or a heap
string.

Add a host test that verifies that no memory is allocated until a
full 11 characters are assigned to a string, as well as checking all
intermediate values.

No user code changes should be required to work with this optimization.

cores/esp8266/StreamString.cpp

@@ -1,66 +1,66 @@
-/**
- StreamString.cpp
-
- Copyright (c) 2015 Markus Sattler. All rights reserved.
- This file is part of the esp8266 core for Arduino environment.
-
- This library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public
- License as published by the Free Software Foundation; either
- version 2.1 of the License, or (at your option) any later version.
-
- This library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with this library; if not, write to the Free Software
- Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
-
- */
-
-#include <Arduino.h>
-#include "StreamString.h"
-
-size_t StreamString::write(const uint8_t *data, size_t size) {
-    if(size && data) {
-        if(reserve(length() + size + 1)) {
-            memcpy((void *) (buffer + len), (const void *) data, size);
-            len += size;
-            *(buffer + len) = 0x00; // add null for string end
-            return size;
-        }
-    }
-    return 0;
-}
-
-size_t StreamString::write(uint8_t data) {
-    return concat((char) data);
-}
-
-int StreamString::available() {
-    return length();
-}
-
-int StreamString::read() {
-    if(length()) {
-        char c = charAt(0);
-        remove(0, 1);
-        return c;
-
-    }
-    return -1;
-}
-
-int StreamString::peek() {
-    if(length()) {
-        char c = charAt(0);
-        return c;
-    }
-    return -1;
-}
-
-void StreamString::flush() {
-}
-
+/**
+ StreamString.cpp
+
+ Copyright (c) 2015 Markus Sattler. All rights reserved.
+ This file is part of the esp8266 core for Arduino environment.
+
+ This library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ This library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with this library; if not, write to the Free Software
+ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+
+ */
+
+#include <Arduino.h>
+#include "StreamString.h"
+
+size_t StreamString::write(const uint8_t *data, size_t size) {
+    if(size && data) {
+        if(reserve(length() + size + 1)) {
+            memcpy((void *) (wbuffer() + len()), (const void *) data, size);
+            setLen(len() + size);
+            *(wbuffer() + len()) = 0x00; // add null for string end
+            return size;
+        }
+    }
+    return 0;
+}
+
+size_t StreamString::write(uint8_t data) {
+    return concat((char) data);
+}
+
+int StreamString::available() {
+    return length();
+}
+
+int StreamString::read() {
+    if(length()) {
+        char c = charAt(0);
+        remove(0, 1);
+        return c;
+
+    }
+    return -1;
+}
+
+int StreamString::peek() {
+    if(length()) {
+        char c = charAt(0);
+        return c;
+    }
+    return -1;
+}
+
+void StreamString::flush() {
+}
+