More consistent style of Array.Extra

[lue-bird]

#4 left some minor code style inconsistencies compared to the other extra modules (many originally introduced by me)

The changes

• replace bs |> (::) a by a :: bs
• replace other single a |> f by f b
• add all the parameters to the function declarations
• annotate let declarations
• consistently catch simple cases early

[gampleman]

I’m ok with this, but I’m personally not that fussed about code style. 🤷

(given the wide range of contributors to the library I think some small variation is inevitable)

I think the main focus of the previous PR was mostly on user visible style, like the way type annotations were done.