Make 'IsDecidable' into a record to aid instance search

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

Lattice/Unit.agda

@@ -7,6 +7,7 @@ open import Data.Unit using (⊤; tt) public
 open import Data.Unit.Properties using (_≟_; ≡-setoid)
 open import Relation.Binary using (Setoid)
 open import Relation.Binary.PropositionalEquality as Eq using (_≡_)
+open import Relation.Binary.Definitions using (Decidable)
 open import Relation.Nullary using (Dec; ¬_; yes; no)
 open import Equivalence
 open import Lattice
@@ -24,9 +25,12 @@ _≈_ = _≡_
     ; ≈-trans = trans
     }
 
-≈-dec : IsDecidable _≈_
+≈-dec : Decidable _≈_
 ≈-dec = _≟_
 
+≈-Decidable : IsDecidable _≈_
+≈-Decidable = record { R-dec = ≈-dec }
+
 _⊔_ : ⊤ → ⊤ → ⊤
 tt ⊔ tt = tt