* lisp/help.el: Rework describe-key's handling of up and double clicks

* lisp/help.el: Rework describe-key's handling of up and double clicks

Use lexical-binding.
(help--binding-undefined-p): New function, extracted from help--analyze-key.
(help--analyze-key): Use it.
(help--filter-info-list): New function.
(describe-key-briefly): Change calling convention.
Handle a list of key sequences now.
(help--binding-locus): Remove unused var 'found'.
(help--read-key-sequence): Rename from help-read-key-sequence.
Almost complete rewrite, with a different return value.
(help-downify-mouse-event-type): Remove.
(describe-key): Change calling convention.
Handle a list of key sequences now.