of_sub_string etc

[cfcs]

We currently have these functions for initializing a new Cstruct:

  val Cstruct.of_bigarray ?off:int -> ?len:int -> buffer -> t

and

  val Cstruct.of_string ?allocator:(int -> t) -> string -> t
  val Cstruct.of_bytes ?allocator:(int -> t) -> bytes -> t
  val Cstruct.of_hex string -> t

It would be nice, and would seem more consistent, if it would be possible to construct a Cstruct.t directly from a slice of a string/bytes (like you can with of_bigarray) instead of having to first do a String.sub/Bytes.sub (which means a redundant memcpy operation) before the copy into the new Cstruct.t.

Also, why is there no ?allocator for of_bigarray? Is that because it doesn't actually copy, but maps to the same memory? If so, that should probably be documented?

[XVilka]

@cfcs slicing was added, can this one be closed then?

[cfcs]

Yes, thanks!

[cfcs]

(I guess the ?allocator question is still relevant, but I have forgotten why I cared when I reported this)