Simplify substitution of let-rec
Simplify substitution of let-rec

The only difference from substitute_var_abs was that the condition had
|| (List.mem vr bound_vars). However, in this case, we can leave the
whole let-rec alone.

matijapretnar/eff,matijapretnar/eff,matijapretnar/eff,matijapretnar/eff,matijapretnar/eff