New procedure (bytevector-zero-explicit! <bv> [<start> [<end>]]).

New procedure (bytevector-zero-explicit! <bv> [<start> [<end>]]).

Intended to zero the memory backing a bytevector even if the values
will never be used again and an aggressively optimizing compiler can
prove that.

Doesn't actually work (GC can move stuff without zeroing it) but it
may help to have it in order to tag where it _would_ be needed if we
could make it work in the future.