Generate epoll bindings

[leostera]

Similar to #4, we need to generate EPoll bindings to be able to drop custom C code from gluon.

The syscalls we need are:

• Create an epoll instance with flags: epoll_create1 : flags:int -> epoll
• Await on an epoll instance: epoll_wait : timeout:int64 -> max_events:int -> epoll -> event array
• Configure an epoll instance: epoll_ctl : epoll -> flags:int -> fd:Fd.t -> event -> unit
• Reset an epoll instance: epoll_ctl_null : epoll -> flags:int -> fd:Fd.t -> unit

The C code we wrote for this lives here.

The generated does not need to match this 1-to-1, so some of the decision I made writing these bindings can be changed, no problem there.

[omnisci3nce]

I'm pretty keen to keep working on bindgen and this seems like a good a way as any to 'dogfood' (😂 ). I can begin working on this if that's okay. No doubt starting on this will spawn a bunch of smaller issues that get us closer one step at a time.

[omnisci3nce]

Could you expand on the caml_enter_blocking_section a tiny bit? I found the section https://v2.ocaml.org/manual/intfc.html on here that talks a bit about it and I think I get it but a little more context might save me wasting a bunch of time. I get the feeling we won't be able to intuit its necessity, if it indeed is necessary, purely from the .h header 🤔