Make composition operator o infix

[s3midetnov]

in src/Category.ard instead of | \fixl 8 o \alias \infixl 8 ∘ {X Y Z : Ob} : Hom Y Z -> Hom X Y -> Hom X Z make | \infixl 8 o \alias \infixl 8 ∘ {X Y Z : Ob} : Hom Y Z -> Hom X Y -> Hom X Z