Support for Effects

[AltGr]

#602 brings us to OCaml 5.1, which has support for effects.
jsoo in turns has support for this feature, but it is disabled by default and requires --enable=effects (enabling it switches to a partial CPS model, which has performance implications)

It'd be nice to try and run with this option, to allow exercises to use and target effects; it may be tricky, some tweaks will probably be needed where we manually apply jsoo-compiled functions.