Make 'IsDecidable' into a record to aid instance search

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

Lattice.agda

@@ -11,8 +11,9 @@ open import Data.Sum using (_⊎_; inj₁; inj₂)
 open import Agda.Primitive using (lsuc; Level) renaming (_⊔_ to _⊔ℓ_)
 open import Function.Definitions using (Injective)
 
-IsDecidable : ∀ {a} {A : Set a} (R : A → A → Set a) → Set a
-IsDecidable {a} {A} R = ∀ (a₁ a₂ : A) → Dec (R a₁ a₂)
+record IsDecidable {a} {A : Set a} (R : A → A → Set a) : Set a where
+    field
+        R-dec : ∀ (a₁ a₂ : A) → Dec (R a₁ a₂)
 
 module _ {a b} {A : Set a} {B : Set b}
     (_≼₁_ : A → A → Set a) (_≼₂_ : B → B → Set b) where