Convert datetime_element_info() to Element classes

Looks great — some early feedback:

1. +++ b/core/modules/datetime/src/Element/Datelist.php
   @@ -0,0 +1,361 @@
   +  public static function validateDatelist(&$element, FormStateInterface $form_state, &$complete_form) {
   
   +++ b/core/modules/datetime/src/Element/Datetime.php
   @@ -0,0 +1,432 @@
   +  public static function validateDatetime(&$element, FormStateInterface $form_state, &$complete_form) {
   

   Any particular reason for using different method names?

   Why don't we consistently use validate()?

2. +++ b/core/modules/datetime/src/Element/Datetime.php
   @@ -0,0 +1,432 @@
   +  public static function processDatetime(&$element, FormStateInterface $form_state, &$complete_form) {
   

   Can we always locate process() above validate(), so that the methods on the element classes are declared in the order in which they are invoked?

   getInfo()
   valueCallback()
   process()
   afterBuild()
   validate()
   preRender()
   postRender()
   

   Also, as above, ditto for processDateTime() → process()