Add simpl never when defining operations

[CohenCyril]

hierarchy-builder/HB/structure.elpi

Line 240 in 8b1725c

pred export-1-operation i:mixinname, i:structure, i:term, i:term, i:one-w-params mixinname, i:option constant, i:list prop, o:list prop.

Right now we accidentally rely on the fact that simpl will not unfold operators of a structure because it has to go through two delta expansions to see the match. We should make this future proof by adding an explicit no simpl.