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/objboundmeth.c b/py/objboundmeth.c
index b961592..c191923 100644
--- a/py/objboundmeth.c
+++ b/py/objboundmeth.c
@@ -38,7 +38,7 @@
 #if MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_DETAILED
 STATIC void bound_meth_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t kind) {
     (void)kind;
-    mp_obj_bound_meth_t *o = o_in;
+    mp_obj_bound_meth_t *o = MP_OBJ_TO_PTR(o_in);
     mp_printf(print, "<bound_method %p ", o);
     mp_obj_print_helper(print, o->self, PRINT_REPR);
     mp_print_str(print, ".");
@@ -48,7 +48,7 @@
 #endif
 
 STATIC mp_obj_t bound_meth_call(mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
-    mp_obj_bound_meth_t *self = self_in;
+    mp_obj_bound_meth_t *self = MP_OBJ_TO_PTR(self_in);
 
     // need to insert self->self before all other args and then call self->meth
 
@@ -77,7 +77,7 @@
         return;
     }
     if (attr == MP_QSTR___name__) {
-        mp_obj_bound_meth_t *o = self_in;
+        mp_obj_bound_meth_t *o = MP_OBJ_TO_PTR(self_in);
         dest[0] = MP_OBJ_NEW_QSTR(mp_obj_fun_get_name(o->meth));
     }
 }
@@ -100,5 +100,5 @@
     o->base.type = &mp_type_bound_meth;
     o->meth = meth;
     o->self = self;
-    return o;
+    return MP_OBJ_FROM_PTR(o);
 }