fix: `grind` using `reducible` transparency setting (#7102)

This PR modifies `grind` to run with the `reducible` transparency
setting. We do not want `grind` to unfold arbitrary terms during
definitional equality tests. This PR also fixes several issues
introduced by this change. The most common problem was the lack of a
hint in proofs, particularly in those constructed using proof by
reflection. This PR also introduces new sanity checks when `set_option
grind.debug true` is used.