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/parse.h b/py/parse.h
index 65ca0f4..b814350 100644
--- a/py/parse.h
+++ b/py/parse.h
@@ -48,7 +48,7 @@
 #define MP_PARSE_NODE_BYTES     (0x0a)
 #define MP_PARSE_NODE_TOKEN     (0x0e)
 
-typedef mp_uint_t mp_parse_node_t; // must be pointer size
+typedef uintptr_t mp_parse_node_t; // must be pointer size
 
 typedef struct _mp_parse_node_struct_t {
     uint32_t source_line;       // line number in source file
@@ -71,7 +71,7 @@
 
 #define MP_PARSE_NODE_LEAF_KIND(pn) ((pn) & 0x0f)
 #define MP_PARSE_NODE_LEAF_ARG(pn) (((mp_uint_t)(pn)) >> 4)
-#define MP_PARSE_NODE_LEAF_SMALL_INT(pn) (((mp_int_t)(pn)) >> 1)
+#define MP_PARSE_NODE_LEAF_SMALL_INT(pn) (((mp_int_t)(intptr_t)(pn)) >> 1)
 #define MP_PARSE_NODE_STRUCT_KIND(pns) ((pns)->kind_num_nodes & 0xff)
 #define MP_PARSE_NODE_STRUCT_NUM_NODES(pns) ((pns)->kind_num_nodes >> 8)