... dynamic switching between direct-style and CPS code. (ocsigen#1461)

Passing a function [f] as argument of `caml_assume_no_effects`
guarantees that, when compiling with `--enable doubletranslate`, the
direct-style version of [f] is called, which is faster than the CPS
version. As a consequence, performing an effect in a transitive callee
of [f] will raise `Effect.Unhandled`, regardless of any effect handlers
installed before the call to `caml_assume_no_effects`, unless a new
effect handler was installed in the meantime.

Usage:

```
external assume_no_effects : (unit -> 'a) -> 'a = "caml_assume_no_effects"

... caml_assume_no_effects (fun () -> (* Will be called in direct style... *)) ...
```

When double translation is disabled, `caml_assume_no_effects` simply
acts like `fun f -> f ()`.

This primitive is exposed via `Js_of_ocaml.Js.Effect.assume_no_perform`.

Co-authored-by: Jérôme Vouillon <[email protected]>

... when double translation is enabled.

We can use a single definition for the CPS transformation and the double
translation. The definition for the CPS transformation was wrong, since
it did not take a continuation as parameter.