Re: [Typer] [Git][monnier/typer][alice] add lexp_lexp' for genericity

• let lp = lexp_lexp' (try OL.lexp_whnf lsort (ectx_to_lctx ctx)

I suspect that many/most calls to `OL.lexp_whnf` will want to pass the result through `lexp_lexp'` before using it, so we could/should either change `OL.lexp_whnf` so it always passes its result through `lexp_lexp'`, or provide two variants of `OL.lexp_whnf` one returning a `lexp` and the other returning lexp'`.

Stefan