Put the fixed point algorithm code into its own file

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

Chain.agda

@@ -6,7 +6,7 @@ module Chain {a} {A : Set a}
              (_R_ : A → A → Set a)
              (R-≈-cong : ∀ {a₁ a₁' a₂ a₂'} → a₁ ≈ a₁' → a₂ ≈ a₂' → a₁ R a₂ → a₁' R a₂') where
 
-open import Data.Nat as Nat using (ℕ; suc; _+_; _≤_)
+open import Data.Nat using (ℕ; suc; _+_; _≤_)
 open import Data.Nat.Properties using (+-comm; m+1+n≰m)
 open import Data.Product using (_×_; Σ; _,_)
 open import Relation.Binary.PropositionalEquality as Eq using (_≡_; refl)