* lisp/gnus/nnmaildir.el: Fix O(n^2) problem when leaving a group

Use lexical-binding.

(nnmaildir-close-group): Use a hash-table rather than a list to keep
track of the files we have seen.

* lisp/gnus/nnheader.el (nnheader-parse-naked-head):
Use make-full-mail-header.
1 job for master in 75 minutes and 4 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #5004
test-all

01:15:04