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.
diff --git a/py/objstringio.h b/py/objstringio.h
new file mode 100644
index 0000000..853bfb1
--- /dev/null
+++ b/py/objstringio.h
@@ -0,0 +1,38 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2016 Damien P. George
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+#ifndef MICROPY_INCLUDED_PY_OBJSTRINGIO_H
+#define MICROPY_INCLUDED_PY_OBJSTRINGIO_H
+
+#include "py/obj.h"
+
+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;
+
+#endif // MICROPY_INCLUDED_PY_OBJSTRINGIO_H