support enough primitives to implement exists and for_all in parallel

[craff]

This typically requires propagating exception and cancellation.

[gasche]

In #90 I propose

val parallel_find : ?chunk_size:int -> start:int -> finish:int ->
  body:(int -> 'a option) -> pool -> 'a option

which lets one easily derive exists, forall but also value-returning versions.

[gasche]

I believe that this can be closed now that #90 is merged (thanks!).