code generation: fewer bindings, do not evaluate configurables multiple times, run initialization once

When I look into a (mirage-)generated main.ml, I find the following code:

let mirage1 = lazy (
  let __noop1 = Lazy.force noop1 in
  let __noop1 = Lazy.force noop1 in
  let __key1 = Lazy.force key1 in
  let __mirage_logs1 = Lazy.force mirage_logs1 in
  let __f11 = Lazy.force f11 in
  __noop1 >>= fun _noop1 ->
  __noop1 >>= fun _noop1 ->
  __key1 >>= fun _key1 ->
  __mirage_logs1 >>= fun _mirage_logs1 ->
  __f11 >>= fun _f11 ->
  Lwt.return_unit
  )

let () =
  let t =
  Lazy.force noop1 >>= fun _ ->
    Lazy.force noop1 >>= fun _ ->
    Lazy.force key1 >>= fun _ ->
    Lazy.force mirage_logs1 >>= fun _ ->
    Lazy.force mirage1
  in run t

Which IMHO has two issues fixed in separate commits in this PR:

all initialization jobs are evaluated twice, once in let () (bound to t), once in mirage1 -- the reason is that Mirage's P (where P.create is called by Functoria) registers the initialization jobs as dependencies -- this is required, otherwise they are not part of functoria's graph, and it complains during configure step (if you can think of another way to handle this, please let me know -- I'd be more in favour of not having to pass init @ jobs to P.create, but instead only jobs, and retain the Lazy.force xxx >>= fun _ in let ())
each configurable is first let-bound to __XX, then Lwt-evaluated __XX >>= fun _X, and then potentially applied to connect, using _X. This is not really intuitive, since now variables which start with an underscore are passed to a function. Furthermore, if depending on the same job multiple times (i.e. noop1 above), these are unneccessarily forced twice and Lwt-evaluated twice.

This patch generates instead the following code:

[@@@ocaml.warning "-27"]

let mirage1 = lazy (
  Lazy.force noop1 >>= fun noop1' ->
  Lazy.force key1 >>= fun key1' ->
  Lazy.force mirage_logs1 >>= fun mirage_logs1' ->
  Lazy.force f11 >>= fun f11' ->
  Lwt.return_unit
  )

let () =
  run (Lazy.force mirage1)

Please let me know what you think (esp. @Drup, but obviously other input welcome) /cc @linse

mirage / functoria

code generation: fewer bindings, do not evaluate configurables multiple times, run initialization once #166