Add precedence to some language constructs

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

Language.agda

@@ -38,6 +38,8 @@ data BasicStmt : Set where
     noop : BasicStmt
 
 infixr 2 _then_
+infix 3 while_repeat_
+infix 3 if_then_else_
 data Stmt : Set where
     ⟨_⟩ : BasicStmt → Stmt
     _then_ : Stmt → Stmt → Stmt