Feature/#9 array functions (#57)

* #9 Added 'APPEND' function

* #9 Added 'FIRST' function

* #9 Added 'FLATTEN' function

* #9 Added 'INTERSECTION' function

* #9 Added 'LAST' function

* #9 Added 'MINUS' function

* #9 Added 'NTH' function

* #9 Added 'OUTERSECTION' function

* #9 Added 'POP' function

* #9 Added 'POSITION' function

* #9 Added 'PUSH' function

* Fixed nil pointer exception in value parser

* #9 Added 'REMOVE_NTH' function

* #9 Added 'REMOVE_VALUE' function

* #9 Added 'REMOVE_VALUES' function

* #9 Added 'REVERSE' function

* #9 Added 'SHIFT' function

* #9 Added 'SLICE' function

* Removed meme

* #9 Added 'SORTED' function

* #9 Added SORTED_UNIQUE function

* #9 Added 'UNION' function

* #9 Added 'UNION_DISTINCT' function

* #9 Added 'UNIQUE' function

* #9 Added 'UNSHIFT' function

* #9 Made more strict optional arg validation

* #9 Fixed linting errors

pkg/stdlib/arrays/shift.go

@@ -0,0 +1,41 @@
+package arrays
+
+import (
+	"context"
+	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+)
+
+/*
+ * Returns a new array without the first element.
+ * @param array (Array) - Target array.
+ * @returns (Array) - Copy of an array without the first element.
+ */
+func Shift(_ context.Context, args ...core.Value) (core.Value, error) {
+	err := core.ValidateArgs(args, 1, 1)
+
+	if err != nil {
+		return values.None, err
+	}
+
+	err = core.ValidateType(args[0], core.ArrayType)
+
+	if err != nil {
+		return values.None, err
+	}
+
+	arr := args[0].(*values.Array)
+
+	length := int(arr.Length())
+	result := values.NewArray(length)
+
+	arr.ForEach(func(value core.Value, idx int) bool {
+		if idx != 0 {
+			result.Push(value)
+		}
+
+		return true
+	})
+
+	return result, nil
+}