fix(frontends/lean/calc_proof_elaborator): bug when inserting symmetry proofs for heq, fixes #286

The problem was that heq type is
    Pi {A : Type} (a : A) {B : Type} (b : B), Prop

The calc_proof_elaborator was assuming that (a : A) (b : B) were the
last two arguments in any relation supported by calc.

The fix is to remove this assumption.