regularize syntax for effect declaration

hello,

(not sure how issues are received at this stage of developpement, i try anyway)

i suggest to change the syntax of effect declaration:

effect E : a -> b

to the more regular:

effect E : a -> b eff

why?

it stays in line with the existing syntax for GADTs and exceptions; in particular the keyword effect can be thought as a textual macro for type _ eff +=.
it simplifies the parser (i guess).
the current syntax is confusing: in effect E : int, E is actually an int eff, not an int; as the type of perform shows, perform E is the promised int.
the current syntax is error‐prone: i have already been bitten several times by two common mistakes, namely:
```
effect E : a eff
```
forgetting that that eff is already appended by the system, which leads to the wrong type a eff eff; and
```
effect E : a -> b -> c
```
which yields a syntax error on the second arrow; i guess the aim to catch the common trapper of the uncurried style for constructors, just as with regular OCaml; but if i think about it, i would rather expect this piece of code te be exactly
```
effect E : a -> (b -> c)
(* i·e· *) type _ eff += E : a -> (b -> c) eff
```
the regular syntax allows the error spotting without discrepancy of that kind.

cons:

(in case it matters: i’m testing effects on the opam switch 4.02.1+multicore, as 4.03.0+effects does not compile for me.)

ocamllabs / ocaml-effects