Be able to mmap outside the Lwt monad

[dinosaure]

It may be interesting to implement the mmap syscall. We don't really need anything very complex and it could just be a simple read() outside the Lwt monad in the first place.

val mmap : t -> off:int64 -> sectors:int64 -> (Cstruct.t list, error) result

It might be interesting to have such access for reading. The proposed interface counts in "sectors". For example off = 1L really means off = 1L * sector_size.

/cc @mirage/core