Use lexical-binding in tempo.el and add tests

Use lexical-binding in tempo.el and add tests

For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-05/msg00395.html
* lisp/tempo.el: Use lexical-binding.
(tempo-define-template): Expand documentation to mention
`tempo-user-elements'.
(tempo-local-tags, tempo-collection, tempo-dirty-collection)
(tempo-marks, tempo-match-finder): Define with defvar-local.
(tempo-named-insertions, tempo-region-start, tempo-region-stop): Make
them automatically buffer-local.
* test/lisp/tempo-tests.el: Add tests for tempo.el.