* lisp/mail/rmailmm.el: Use `cl-defstruct` and `lexical-binding`

* lisp/mail/rmailmm.el: Use `cl-defstruct` and `lexical-binding`

Remove redundant `:group` args.
(rmail-mime-entity): Make it a `cl-defstruct`.
(rmail-mime-entity-set-truncated): Mark as obsolete.
(rmail-mime-display): New `cl-defstruct`.
(rmail-mime-shown-mode, rmail-mime-hidden-mode, rmail-mime-raw-mode)
(rmail-mime-toggle-hidden, rmail-mime-update-tagline)
(rmail-mime-text-handler, rmail-mime-bulk-handler)
(rmail-mime-process-multipart, rmail-mime-handle, rmail-mime-process)
(rmail-mime-parse, rmail-mime-insert, rmail-show-mime): Adjust accordingly.
(rmail-mime-toggle-raw): Apply de Morgan.
(rmail-mime-insert-text): Remove unused var `tagline`.
(rmail-mime-insert-image): Remove unused var `content-type`.
(shr-inhibit-images, shr-width): Declare vars.
(rmail-mime-insert-multipart): Remove unused vars `tagline` and `body`.
(rmail-mime-insert): Remove unused var `tagline`.
(rmail-search-mime-message): Remove unused var `body-end`.