Put call to qstr_init and mp_init_emergency_exc_buf in mp_init.

qstr_init is always called exactly before mp_init, so makes sense to
just have mp_init call it.  Similarly with
mp_init_emergency_exception_buf.  Doing this makes the ports simpler and
less error prone (ie they can no longer forget to call these).

unix/main.c

@@ -278,11 +278,7 @@ int main(int argc, char **argv) {
     char *heap = malloc(heap_size);
     gc_init(heap, heap + heap_size);
 #endif
-#if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF
-    mp_init_emergency_exception_buf();
-#endif
 
-    qstr_init();
     mp_init();
 
     char *home = getenv("HOME");