`R.prepend`/`R.append` fns accept broader types

[Peeja]

Before, R.prepend(4) or R.append(4) would produce a function which could only be applied to a number[]. However, you may well want to append a 4 to an array whose elements can be numbers or other things, such as a (string | number)[]. This commit improves the types to accept such arrays.

You may also have an array of a more specific type (such as number[]) and wish to use R.prepend/R.append to expand the type. Because it's usually a mistake, append("d")(listOfNumbers) will cause a type error; however, explicitly calling append("d")<string | number>(listOfNumbers) will give you a (string | number)[].

[selfrefactor]

Thanks for this PR.

I'd change few things before releasing:

• Introducing TElement/TNewElement while correct in the context of R.append deviates from the rest of types, so I will make it more generic.

• I'll check again if change in R.zipObj test is required.

[Peeja]

Sounds good! I went with that sort of naming because I kept losing track of what was what otherwise, but please do adjust it as you see fit. 🙂