[python] Enable PYSTACK for emscripten platform: this "fixes" bug

probably due to gc_collect (we suspect that transpiled C does not have
access to javascript variables preventing it from collecting all
required roots and leading to deleting objects prematuraly). Enabling
PyStack reduces the use of the heap and fixes the bug.

python/port/port.cpp

@@ -102,6 +102,11 @@ extern "C" {
 }
 
 void MicroPython::init(void * heapStart, void * heapEnd) {
+#if __EMSCRIPTEN__
+  static mp_obj_t pystack[1024];
+  mp_pystack_init(pystack, &pystack[MP_ARRAY_SIZE(pystack)]);
+#endif
+
   volatile int stackTop;
   void * stackTopAddress = (void *)(&stackTop);
   /* We delimit the stack part that will be used by Python. The stackTop is the