The main thrust is to get rid of the use of the `cl-struct-slots'
property which doesn't exist in Emacs-25 any more.
* yasnippet.el: Update copyright
(yas--template): Change constructor name from
`yas--make-blank-template' to `yas--make-template'
(yas--populate-template): Remove.
(yas--define-snippets-1): Use `yas--make-template'
(yas--define-snippets-1): Use `setf's
(yas-define-menu, yas--define-menu-1, yas-tryout-snippet): Use
`yas--make-template'