py: Support conversion of bignum to bytes.

This gets int.to_bytes working for bignum, and also struct.pack with 'q'
and 'Q' args on 32-bit machines.

Addresses issue #1155.

py/binary.c

@@ -32,6 +32,7 @@
 
 #include "py/binary.h"
 #include "py/smallint.h"
+#include "py/objint.h"
 
 // Helpers to work with binary-encoded data
 
@@ -282,10 +283,13 @@ void mp_binary_set_val(char struct_type, char val_type, mp_obj_t val_in, byte **
         }
 #endif
         default:
-            // we handle large ints here by calling the truncated accessor
+            #if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE
             if (MP_OBJ_IS_TYPE(val_in, &mp_type_int)) {
-                val = mp_obj_int_get_truncated(val_in);
-            } else {
+                mp_obj_int_to_bytes_impl(val_in, struct_type == '>', size, p);
+                return;
+            } else
+            #endif
+            {
                 val = mp_obj_get_int(val_in);
             }
     }