bigstring intrinsics and fast blits for OCaml (dev files)

 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.
 .
 This package contains development files.