Bot: v2: AddMiddleware to use Go methods instead of arbitrary strings

[diamondburned]

Preamble

As of right now, the AddMiddleware API takes in an arbitrary string for method names. This is because at the time of writing this API, I did not know that method names could be taken by passing the method function in.

Code example:

func main() {
	var t T
	printMethod(t.Method_dash)
}

type T struct{}

func (T) Method_dash() {}

func printMethod(v interface{}) {
	ptr := reflect.ValueOf(v).Pointer()
	fmt.Println(runtime.FuncForPC(ptr).Name()) // Output: main.T.Method_dash-fm
}

As the example's output shows, some string manipulation will be needed.

Proposal

This issue proposes the new AddMiddleware API to have this function signature:

func (sub *Subcommand) AddMiddleware(methodName, middleware interface{})

To preserve backwards compatibility, AddMiddleware will use reflect.Kind to ensure that the passed in type is only either a string or a function. If a string is provided, the old behavior will run. If a function is provided, the library will use the above method to get the method name, then execute the old behavior with that name. If any other type is given, the function will panic.

An example of this API would be:

func (t *Tags) Setup(sub *bot.Subcommand) {
	sub.AddMiddleware(t.A, func() {}) // new behavior
	sub.AddMiddleware("B", func() {}) // old behavior
}

func (t *Tags) A(*gateway.MessageCreateEvent) {}
func (t *Tags) B(*gateway.MessageCreateEvent) {}