Add @impl (#104)

Per https://elixir-lang.org/getting-started/typespecs-and-behaviours.html#adopting-behaviours, it allows to get more warnings in case the implementation does not respect the behaviour anymore.

lib/saxy/simple_form/handler.ex

@@ -3,15 +3,18 @@ defmodule Saxy.SimpleForm.Handler do
 
   @behaviour Saxy.Handler
 
+  @impl Saxy.Handler
   def handle_event(:start_document, _prolog, stack) do
     {:ok, stack}
   end
 
+  @impl Saxy.Handler
   def handle_event(:start_element, {tag_name, attributes}, stack) do
     tag = {tag_name, attributes, []}
     {:ok, [tag | stack]}
   end
 
+  @impl Saxy.Handler
   def handle_event(:characters, chars, stack) do
     [{tag_name, attributes, content} | stack] = stack
 
@@ -20,6 +23,7 @@ defmodule Saxy.SimpleForm.Handler do
     {:ok, [current | stack]}
   end
 
+  @impl Saxy.Handler
   def handle_event(:cdata, chars, stack) do
     [{tag_name, attributes, content} | stack] = stack
 
@@ -28,6 +32,7 @@ defmodule Saxy.SimpleForm.Handler do
     {:ok, [current | stack]}
   end
 
+  @impl Saxy.Handler
   def handle_event(:end_element, tag_name, [{tag_name, attributes, content} | stack]) do
     current = {tag_name, attributes, Enum.reverse(content)}
 
@@ -42,6 +47,7 @@ defmodule Saxy.SimpleForm.Handler do
     end
   end
 
+  @impl Saxy.Handler
   def handle_event(:end_document, _, stack) do
     {:ok, stack}
   end