misc lemmas min/max/clamp real numbers

[malarbol]

This PR introduces a few lemmas on the min/max/clamp functions on real numbers:

• for any x : ℝ, the functions max-ℝ x and min-ℝ x are increasing;
• for any closed interval I, the clamping map clamp-I is increasing and strictly increasing on I;

We also prove the following location property for proper closed intervals: for any proper closed interval [a, b] and rational numbers p < q, then either:

• p < a;
• q < b;
• there exists (s r : ℚ) such that a < s < r < b and p < s < r < q.