Support bs_init_writable

[bjfish]

This is mentioned here:
https://github.com/aggelgian/cuter/blob/6723f5e67de4d309aa198fd44b84e530cdb961de/src/cuter_eval.erl#L677

[kostis]

Thanks for opening this issue. A small unit test that shows the need for the support, and its "expected" outcome, will help prioritizing this -- esp. if it is a pointer to some existing code you want to test.