feat(frontends/lean/calc): expose get_calc_subst_info and get_calc_refl_info APIs
feat(frontends/lean/calc_proof_elaborator): add '{...}' if needed in calc proof steps

This is part of #268
feat(frontends/lean/calc_proof_elaborator): add '{...⁻¹}' if needed in calc proofs, closes #268

This commit also simplifies library/data/nat/basic.lean