OCaml bindings to liburing

much fast, very wip

TODO

• prepend function names with prepare_ (to prevent name collisions when adding io_uring_prep_close)
• port more examples from liburing
• add option to use writev/readv in cp example
• add support for request linking (and add to cp example)
  • note: at first, link-cp.c seems broken since there doesn't seem to be a way to read+write the reminder when one of the two syscalls read/writes less then expected, but seems like checking for failure and re-submitting works? (c.f. axboe/liburing#58)
• add support for submission queue polling
• add support for fixed buffers
• add benchmarks
• functorize interface to implement a version of the API that explicitly checks whether the kernel supports the relevant functions (similar to Bigstring_unix.recvmmsg_assume_fd_is_nonblocking)
  • io_uring_get_probe()
  • change 'a t to ('a, 'witness) t to prevent cross-instance usage?
    • idea: ensure only one version exists with something like Io_uring.Make()
• add tests
• create default value with Obj.magic and check for emptiness in user_data array via phys_equal in invariant
• properly allocate io_uring
• test prep_send/prep_recv with httpaf?
• check for errno in library
• add ``Wait_for_num_completions of inttoIo_uring.wait`
• allocate Queued_sockaddr on the heap
• add buffer provision support (and add it in echo server)
• keep track of in-flight requests and apply backpressure so completion queue never overflows?
  • backpressure is interesting. Even though we can straightforwardly keep track of in-flight requests and make sure we never submit more than the number of free slots in the completion queue, this may result in starvation when we submit many requests which may have unbounded completion times (like accept or poll). We probably need to think about IORING_SETUP_CQ_NODROP and clearly document behavior around this.
• Use the foreign build sandboxing approach in dune to fix the liburing version?