Enhancement: Add array.slice function

[teq0]

#851 mentions both slice and concat operations on arrays. array.concat was added and #851 was closed, but I believe there is still a valid need for slice.

[Infi-Knight]

I know this was supposed to be a "low-hanging-fruit" but I am stuck and would really appreciate some help:
Going through the code I think that I need to follow these steps:

1. Register ArraySlice in ast/builtins.go
2. Implement and register builtinArraySlice in topdown/array.go
3. Write requisite tests and make sure it works

For the first step I wrote following code:

//Arrays
ArrayConcat,
ArraySlice


// ArraySlice returns a slice of a given array
var ArraySlice = &Builtin{
	Name: "array.slice",
	Decl: types.NewFunction(
		types.Args(
			types.NewArray(nil, types.A),
			types.NewNumber(),
			types.NewNumber(),
		),
		types.NewArray(nil, types.A),
	),
}

For the second step, I have setup the following skeleton:

func builtinArraySlice(a, i, j ast.Value) (ast.Value, error) {

	// initialize required variables

	// arrA, err := builtins.ArrayOperand(a, 1)
	// if err != nil {
	// 	return nil, err
	// }

	startIndex, err := builtins.IntOperand(i, 2)
	if err != nil {
		return nil, err
	}

	stopIndex, err := builtins.IntOperand(j, 3)
	if err != nil {
		return nil, err
	}

	arrB := make(ast.Array, 0, stopIndex-startIndex)

	// implement the logic for slicing

	return arrB, nil

}

Now when I try to register this new function:

func init() {
	RegisterFunctionalBuiltin2(ast.ArrayConcat.Name, builtinArrayConcat)
	RegisterFunctionalBuiltin2(ast.ArraySlice.Name, builtinArraySlice)
}

editor(vscode) hints
following error:

cannot use builtinArraySlice (type func(ast.Value, ast.Value, ast.Value) (ast.Value, error)) as type FunctionalBuiltin2 in argument to RegisterFunctionalBuiltin2

What am I doing wrong here?

[tsandall]

@Infi-Knight You'll need to use RegisterFunctionalBuiltin3 instead of RegisterFunctionalBuiltin2 here. The former takes accepts 3 parameters while the latter only accepts 2.