* lisp/gnus/mm-decode.el: Use lexical-binding and use cl-lib
(mm-display-parts): Remove unused arg 'no-default'. Use 'cond'.
(mm-display-external): Use closures rather than `(lambda ...).
Don't bother with 'lexical-let'.
(mm-insert-part): No need for string-to-multibyte now that
'insert' will do that for us now (it used to behave more like
string-make-multibyte).
(mm-pipe-part): Remove unused var 'name'.
(shr-width, shr-content-function, shr-inhibit-images): Declare.
(mm-shr): Use a closure rather than `(lambda ...).