extmod/modujson: Implement ujson.load() to load JSON from a stream.

This refactors ujson.loads(s) to behave as ujson.load(StringIO(s)).

Increase in code size is: 366 bytes for unix x86-64, 180 bytes for
stmhal, 84 bytes for esp8266.

py/objstringio.c

@@ -30,18 +30,12 @@
 
 #include "py/nlr.h"
 #include "py/objstr.h"
+#include "py/objstringio.h"
 #include "py/runtime.h"
 #include "py/stream.h"
 
 #if MICROPY_PY_IO
 
-typedef struct _mp_obj_stringio_t {
-    mp_obj_base_t base;
-    vstr_t *vstr;
-    // StringIO has single pointer used for both reading and writing
-    mp_uint_t pos;
-} mp_obj_stringio_t;
-
 #if MICROPY_CPYTHON_COMPAT
 STATIC void check_stringio_is_open(const mp_obj_stringio_t *o) {
     if (o->vstr == NULL) {