Add extra map, bind and join operators.

[tmcgilchrist]

What do you think about adding some more fundamental operators to this library @talex5 ?
In writing some documentation on using this library outside of ocurrent I ended up defining the following functions.

val map2 : ?eq:('c -> 'c -> bool) -> ('a -> 'b -> 'c) -> 'a t -> 'b t -> 'c t
val bind : ?eq:('b -> 'b -> bool) -> ('a -> 'b t) -> 'a t -> 'b t
val join : ?eq:('a -> 'a -> bool) -> 'a t t -> 'a t

Then you can write code like this in a more natural style.

let rec merge ar ~f =
    if Array.length ar <= 1 then ar.(0)
    else
      let len = Array.length ar in
      let len' = len / 2 + (len mod 2) in
      let ar' =
        Array.init len' (fun i ->
          if i * 2 + 1 >= len then ar.(i*2)
          else Current_incr.map2 f ar.(i*2) ar.(i*2+1))
      in
      merge ar' ~f
      
let average ar =
  let sum = merge ar ~f:(+.) in
  Current_incr.map (fun s -> s /. float (Array.length ar)) sum      
 
 let average_of_prefix ar length =
    Current_incr.bind (fun length ->
      average (Array.init length (fun i -> ar.(i)))) length 

[talex5]

If you're going to add that, it might be better to use the new let syntax instead, e.g.

let average ar =
  let sum = merge ar ~f:(+.) in
  Current_incr.map (fun s -> s /. float (Array.length ar)) sum 

becomes

let average ar =
  let+ sum = merge ar ~f:(+.) in
  sum /. float (Array.length ar))

Not sure it's worth it, given how small the library is at present, but we did already add map.

https://discuss.ocaml.org/t/ann-current-incr-a-small-incremental-library-with-no-dependencies/5531/7?u=talex5 notes that writing code this way may be a little less efficient.