ocaml-bigstringaf 0.8.0 Bigstring intrinsics and fast blits based on memcpy/memmove

The OCaml compiler has a bunch of intrinsics for Bigstrings, but they're not widely-known, sometimes misused, and so programs that use Bigstrings are slower than they have to be. And even if a library got that part right and exposed the intrinsics properly, the compiler doesn't have any fast blits between Bigstrings and other string-like types. bigstringaf provides these missing pieces.