Support some Unicode syntax

[fizruk]

Specifically the following Unicode syntax is added and is used by default when pretty-printing:

• I × J (use \x or \times) for cube products (ASCII version: I * J)
• ⊤ (use \top) for the top tope (ASCII version: TOP)
• ⊥ (use \bot) for the bottom tope (ASCII version: BOT)
• t ≡ s (use \equiv) for point equality tope (ASCII version: t === s)
• t ≤ s (use \leq or (⌥+<) on macOS) for point inequality tope (ASCII version: t <= s)
• φ ∧ ψ (use \and) for tope conjunction (ASCII version: phi /\ psi)
• φ ∨ ψ (use \or) for tope disjunction (ASCII version: phi \/ psi)
• A → B (use \to) for function types (ASCII version: A -> B)
• \ t → s (use \to) for lambda abstraction (ASCII version: \ t -> s)
• π₁ t and π₂ t (use \pi\1 and \pi\2) for projections (ASCII versions: first t and second t)
• φ ↦ a (use \mapsto) for mappings used in extension types and recOR (ASCII version: phi |-> a)
• *₁ (use \1) for the only point in the unit cube (ASCII version: *_1)
• 0₂ (use \2) for point 0 in the directed interval 2 (ASCII version: 0_2)
• 1₂ (use \2) for point 1 in the directed interval 2 (ASCII version: 1_2)

The following syntax is NOT added on purpose:

• λ x → t — we want to keep λ as a valid variable name;

The following syntax was already supported but will now also be used by the pretty-printer:

• Σ (x : A), B x (use \Sigma) for the dependent sum types

The following syntax is marked as deprecated with warnings issued via Debug.Trace:

• recOR(ψ, φ, a_ψ, a_φ) is deprecated since the order of arguments is ad hoc; instead recOR(ψ ↦ a_ψ, φ ↦ a_φ) should be used
• The use of angle brackets (< and >) in extension types <{t : I | ψ} → A t [ φ t ↦ a t ]> is now deprecated (it actually was deprecated since v0.2.0) since it is now a compound type anyway; instead, omit the angle brackets, possibly replacing them with parentheses.