Copy ESP8266 String w/SSO to ESP32 repo (#2715)

I redid the ESP8266 WString library to enable small string optimization
(SSO) a while back, and think it would be helpful even on the ESP32 with
its higher memory complement.

SSO avoids lots of tiny mallocs() on the heap which cause fragmentation
by using the memory in the class object itself to store the actual
string and only mallocing() for buffers that are larger than what can
fit in thie class object.  Modern C++ std::string implementations have
this optimization as well, but since we're using Arduino strings we had
to roll our own.

cores/esp32/StreamString.cpp

@@ -2,6 +2,7 @@
  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
@@ -22,31 +23,27 @@
 #include <Arduino.h>
 #include "StreamString.h"
 
-size_t StreamString::write(const uint8_t *data, size_t size)
-{
+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
+            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)
-{
+size_t StreamString::write(uint8_t data) {
     return concat((char) data);
 }
 
-int StreamString::available()
-{
+int StreamString::available() {
     return length();
 }
 
-int StreamString::read()
-{
+int StreamString::read() {
     if(length()) {
         char c = charAt(0);
         remove(0, 1);
@@ -56,8 +53,7 @@ int StreamString::read()
     return -1;
 }
 
-int StreamString::peek()
-{
+int StreamString::peek() {
     if(length()) {
         char c = charAt(0);
         return c;
@@ -65,7 +61,6 @@ int StreamString::peek()
     return -1;
 }
 
-void StreamString::flush()
-{
+void StreamString::flush() {
 }