objstr: Allow to define statically allocated str objects.

Similar to tuples, lists, dicts. Statically allocated strings don't have hash
computed.

py/objstr.h

@@ -0,0 +1,10 @@
+typedef struct _mp_obj_str_t {
+    mp_obj_base_t base;
+    // XXX here we assume the hash size is 16 bits (it is at the moment; see qstr.c)
+    machine_uint_t hash : 16;
+    // len == number of bytes used in data, alloc = len + 1 because (at the moment) we also append a null byte
+    machine_uint_t len : 16;
+    const byte *data;
+} mp_obj_str_t;
+
+#define MP_DEFINE_STR_OBJ(obj_name, str) mp_obj_str_t obj_name = {{&mp_type_str}, 0, sizeof(str) - 1, (const byte*)str};