py: Wrap all obj-ptr conversions in MP_OBJ_TO_PTR/MP_OBJ_FROM_PTR.

This allows the mp_obj_t type to be configured to something other than a
pointer-sized primitive type.

This patch also includes additional changes to allow the code to compile
when sizeof(mp_uint_t) != sizeof(void*), such as using size_t instead of
mp_uint_t, and various casts.
diff --git a/py/objstringio.c b/py/objstringio.c
index d0c0a4f..5a8f43d 100644
--- a/py/objstringio.c
+++ b/py/objstringio.c
@@ -54,13 +54,13 @@
 
 STATIC void stringio_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t kind) {
     (void)kind;
-    mp_obj_stringio_t *self = self_in;
+    mp_obj_stringio_t *self = MP_OBJ_TO_PTR(self_in);
     mp_printf(print, self->base.type == &mp_type_stringio ? "<io.StringIO 0x%x>" : "<io.BytesIO 0x%x>", self);
 }
 
 STATIC mp_uint_t stringio_read(mp_obj_t o_in, void *buf, mp_uint_t size, int *errcode) {
     (void)errcode;
-    mp_obj_stringio_t *o = o_in;
+    mp_obj_stringio_t *o = MP_OBJ_TO_PTR(o_in);
     check_stringio_is_open(o);
     mp_uint_t remaining = o->vstr->len - o->pos;
     if (size > remaining) {
@@ -73,7 +73,7 @@
 
 STATIC mp_uint_t stringio_write(mp_obj_t o_in, const void *buf, mp_uint_t size, int *errcode) {
     (void)errcode;
-    mp_obj_stringio_t *o = o_in;
+    mp_obj_stringio_t *o = MP_OBJ_TO_PTR(o_in);
     check_stringio_is_open(o);
     mp_uint_t remaining = o->vstr->alloc - o->pos;
     if (size > remaining) {
@@ -93,14 +93,14 @@
 #define STREAM_TO_CONTENT_TYPE(o) (((o)->base.type == &mp_type_stringio) ? &mp_type_str : &mp_type_bytes)
 
 STATIC mp_obj_t stringio_getvalue(mp_obj_t self_in) {
-    mp_obj_stringio_t *self = self_in;
+    mp_obj_stringio_t *self = MP_OBJ_TO_PTR(self_in);
     check_stringio_is_open(self);
     return mp_obj_new_str_of_type(STREAM_TO_CONTENT_TYPE(self), (byte*)self->vstr->buf, self->vstr->len);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(stringio_getvalue_obj, stringio_getvalue);
 
 STATIC mp_obj_t stringio_close(mp_obj_t self_in) {
-    mp_obj_stringio_t *self = self_in;
+    mp_obj_stringio_t *self = MP_OBJ_TO_PTR(self_in);
 #if MICROPY_CPYTHON_COMPAT
     vstr_free(self->vstr);
     self->vstr = NULL;
@@ -122,7 +122,7 @@
 
 STATIC mp_obj_stringio_t *stringio_new(mp_obj_t type_in) {
     mp_obj_stringio_t *o = m_new_obj(mp_obj_stringio_t);
-    o->base.type = type_in;
+    o->base.type = MP_OBJ_TO_PTR(type_in);
     o->vstr = vstr_new();
     o->pos = 0;
     return o;
@@ -135,11 +135,11 @@
     if (n_args > 0) {
         mp_buffer_info_t bufinfo;
         mp_get_buffer_raise(args[0], &bufinfo, MP_BUFFER_READ);
-        stringio_write(o, bufinfo.buf, bufinfo.len, NULL);
+        stringio_write(MP_OBJ_FROM_PTR(o), bufinfo.buf, bufinfo.len, NULL);
         // Cur ptr is always at the beginning of buffer at the construction
         o->pos = 0;
     }
-    return o;
+    return MP_OBJ_FROM_PTR(o);
 }
 
 STATIC const mp_rom_map_elem_t stringio_locals_dict_table[] = {
@@ -174,7 +174,7 @@
     .getiter = mp_identity,
     .iternext = mp_stream_unbuffered_iter,
     .stream_p = &stringio_stream_p,
-    .locals_dict = (mp_obj_t)&stringio_locals_dict,
+    .locals_dict = (mp_obj_dict_t*)&stringio_locals_dict,
 };
 
 #if MICROPY_PY_IO_BYTESIO
@@ -186,7 +186,7 @@
     .getiter = mp_identity,
     .iternext = mp_stream_unbuffered_iter,
     .stream_p = &bytesio_stream_p,
-    .locals_dict = (mp_obj_t)&stringio_locals_dict,
+    .locals_dict = (mp_obj_dict_t*)&stringio_locals_dict,
 };
 #endif