Switch maps (and consequently most of the code) to using instances

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

Lattice/Unit.agda

@@ -28,8 +28,9 @@ _≈_ = _≡_
 ≈-dec : Decidable _≈_
 ≈-dec = _≟_
 
-≈-Decidable : IsDecidable _≈_
-≈-Decidable = record { R-dec = ≈-dec }
+instance
+    ≈-Decidable : IsDecidable _≈_
+    ≈-Decidable = record { R-dec = ≈-dec }
 
 _⊔_ : ⊤ → ⊤ → ⊤
 tt ⊔ tt = tt