Improve library facilities for handling `'a option tzresult Lwt.t`s

It's very common to see 'a option tzresult Lwt.ts, and they're tricky to work with as a result of not having any functions specific to them.

For example, the following pattern is pervasive; suppose we have in our context

  type a
  type b
  val mx: a option tzresult Lwt.t
  val foo: a -> b tzresult Lwt.t
  (* partial or impure *)
  val fallback: unit -> b tzresult Lwt.t

then we might wish to evaluate

let bar: b tzresult Lwt.t =
  mx >>=? function
  | Some x -> foo x
  | None -> fallback()

This could be addressed with a carefully thought out library for 'a option tzresult Lwt.t. A submodule Error_monad.Option might be a good idea.

for example, if we might have something like had

module Error_monad
...
module Option : sig
  val (>>?): 'a option tzresult -> ('a -> 'b option tzresult) -> 'b option tzresult
  val (>>=?): 'a option tzresult Lwt.t -> ('a -> 'b option tzresult Lwt.t) -> 'b option tzresult Lwt.t

  (* The mapping is `'a -> 'b tzresult` because we don't eliminate `tzresult` *)
  val unopt_map_lazy:
    f:('a -> 'b tzresult)
    -> default:(unit -> 'b tzresult)
    -> 'a option tzresult
    -> 'b tzresult

  (* The mapping is `'a -> 'b tzresult Lwt.t` because we don't eliminate `tzresult` or `Lwt.t` *)
  module Lwt: sig
    val unopt_map_lazy:
      f:('a -> 'b tzresult Lwt.t)
      -> default:(unit -> 'b tzresult Lwt.t)
      -> 'a option tzresult Lwt.t
      -> 'b tzresult Lwt.t
  end
end

then the example would become

let bar: b tzresult Lwt.t =
  Error_monad.Option.Lwt.unopt_map_lazy ~f:foo ~default:fallback mx