all: Rename BYTES_PER_WORD to MP_BYTES_PER_OBJ_WORD.

The "word" referred to by BYTES_PER_WORD is actually the size of mp_obj_t
which is not always the same as the size of a pointer on the target
architecture.  So rename this config value to better reflect what it
measures, and also prefix it with MP_.

For uses of BYTES_PER_WORD in setting the stack limit this has been
changed to sizeof(void *), because the stack usually grows with
machine-word sized values (eg an nlr_buf_t has many machine words in it).

Signed-off-by: Damien George <damien@micropython.org>

py/persistentcode.c

@@ -595,7 +595,7 @@ STATIC void mp_print_bytes(mp_print_t *print, const byte *data, size_t len) {
     print->print_strn(print->data, (const char *)data, len);
 }
 
-#define BYTES_FOR_INT ((BYTES_PER_WORD * 8 + 6) / 7)
+#define BYTES_FOR_INT ((MP_BYTES_PER_OBJ_WORD * 8 + 6) / 7)
 STATIC void mp_print_uint(mp_print_t *print, size_t n) {
     byte buf[BYTES_FOR_INT];
     byte *p = buf + sizeof(buf);