Make slice wrapping pattern more consistent

and move *Args out into its own file

context.go

@@ -157,59 +157,6 @@ func (c *Context) NArg() int {
 	return c.Args().Len()
 }
 
-// Args wraps a string slice with some convenience methods
-type Args struct {
-	slice []string
-}
-
-// Get returns the nth argument, or else a blank string
-func (a *Args) Get(n int) string {
-	if len(a.slice) > n {
-		return a.slice[n]
-	}
-	return ""
-}
-
-// First returns the first argument, or else a blank string
-func (a *Args) First() string {
-	return a.Get(0)
-}
-
-// Tail returns the rest of the arguments (not the first one)
-// or else an empty string slice
-func (a *Args) Tail() []string {
-	if a.Len() >= 2 {
-		return a.slice[1:]
-	}
-	return []string{}
-}
-
-// Len returns the length of the wrapped slice
-func (a *Args) Len() int {
-	return len(a.slice)
-}
-
-// Present checks if there are any arguments present
-func (a *Args) Present() bool {
-	return a.Len() != 0
-}
-
-// Swap swaps arguments at the given indexes
-func (a *Args) Swap(from, to int) error {
-	if from >= a.Len() || to >= a.Len() {
-		return errors.New("index out of range")
-	}
-	a.slice[from], a.slice[to] = a.slice[to], a.slice[from]
-	return nil
-}
-
-// Slice returns a copy of the internal slice
-func (a *Args) Slice() []string {
-	ret := make([]string, len(a.slice))
-	copy(ret, a.slice)
-	return ret
-}
-
 func lookupFlagSet(name string, ctx *Context) *flag.FlagSet {
 	for _, c := range ctx.Lineage() {
 		if f := c.flagSet.Lookup(name); f != nil {