Appveyor support for ESP32

[gregjesl]

In one of the Linux builds in Appveyor, the build should also attempt to build Simpleson as an ESP32 library.

[gregjesl]

ESP build sequence produced the following warnings:

/media/psf/Home/Documents/GitHub/simpleson/json.cpp:238:24: warning: this statement may fall through [-Wimplicit-fallthrough=]
  238 |         this->set_state(STRING_OPEN);
      |         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:239:5: note: here
  239 |     case STRING_OPEN:
      |     ^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp: In member function 'json::reader::push_result json::reader::push_array(char)':
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:321:24: warning: this statement may fall through [-Wimplicit-fallthrough=]
  321 |         this->set_state(ARRAY_READING_VALUE);
      |         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:323:5: note: here
  323 |     case ARRAY_READING_VALUE:
      |     ^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:326:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  326 |         switch (this->sub_reader->push(next))
      |         ^~~~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:355:5: note: here
  355 |     case ARRAY_AWAITING_NEXT_LINE:
      |     ^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp: In member function 'json::reader::push_result json::reader::push_object(char)':
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:380:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  380 |         if(next == '}') {
      |         ^~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:386:5: note: here
  386 |     case OBJECT_AWAITING_NEXT_LINE:
      |     ^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:403:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  403 |         switch (this->sub_reader->push(next))
      |         ^~~~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:432:5: note: here
  432 |     case OBJECT_CLOSED:
      |     ^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp: In member function 'json::reader::push_result json::reader::push_number(char)':
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:464:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  464 |         if(IS_DIGIT(next)) {
      |         ^~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:469:5: note: here
  469 |     case NUMBER_ZERO:
      |     ^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:505:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  505 |         if(next == '+' || next == '-') {
      |         ^~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:511:5: note: here
  511 |     case NUMBER_EXPONENT_SIGN:
      |     ^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp: In member function 'json::reader::push_result json::reader::push_null(char)':
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:603:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  603 |         if(next == 'n') {
      |         ^~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:607:5: note: here
  607 |     case 1:
      |     ^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp: In function 'std::string json::parsing::decode_string(const char*)':
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:741:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
  741 |                 index += 4;
      |                 ~~~~~~^~~~
/media/psf/Home/Documents/GitHub/simpleson/json.cpp:742:13: note: here
  742 |             default:
      |             ^~~~~~~

[gregjesl]

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