(imap-digest-md5-auth, imap-cram-md5-auth): Use `base64-encode-string' and

(imap-digest-md5-auth, imap-cram-md5-auth): Use `base64-encode-string' and
`base64-decode-string' instead of `imap-base64-encode-string' or
`imap-base64-decode-string'.
(base64-encode-string): New function.  It won't be defined if it is already
bound and the optional second arg is allowed.
(base64-decode-string): New function defined by `defun-maybe'.
(imap-base64-encode-string, imap-base64-decode-string): Remove.
(mel-find-function): Autoload "mel".