Emacs occasionally loads Elisp files just to get more info (e.g. for C-h f),
so loading a file should "no effect".  Fix the most obvious such effects:
the splash screen and the autotests by moving those effects into a function.

* coq/coq-autotest.el: Make it declarative.  Use lexical-binding.
(coq-autotest): New function holding the code that used to be at top-level.

* generic/proof.el: Use lexical-binding.
Don't call proof-splash-message just because we're being loaded.

* generic/proof-script.el: Use lexical-scoping; fix all warnings.
(pg-show-all-portions): Simplify the code with a closure.
(proof-activate-scripting): Declare activated-interactively as dyn-scoped.
(proof--splash-done): New var.
(proof-mode): Call proof-splash-message upon first use.

* generic/proof-splash.el (proof-splash-message): Don't check
byte-compile-current-file now that we're only called when the mode
is activated.

* acl2/acl2.el (auto-mode-alist): Use `add-to-list` and \'.

* coq/coq-db.el (coq-build-menu-from-db-internal): Avoid silly O(N²).

* coq/coq-seq-compile.el:
* coq/coq-par-test.el:
* coq/coq-par-compile.el: Fix leftover uses of CL's `assert`.

* generic/proof-utils.el:
* generic/pg-movie.el:
* etc/testsuite/pg-test.el:
* coq/coq-syntax.el: Fix leftover uses of CL's `incf`.

* generic/pg-autotest.el: Fix leftover uses of CL's `decf`.

* obsolete/plastic/plastic.el (plastic-preprocessing): Fix leftover use
of CL's `loop`.

* generic/pg-user.el (proof-add-completions): Do nothing if no
proof-assistant is set yet (i.e. during byte-compilation).
(byte-compile-current-file): No need to test this any more.

* generic/proof-syntax.el (proof-regexp-alt-list): Use mapconcat.
Remove unnecessary "\\(?:...\\)".
(proof-regexp-alt): Redefine in terms of proof-regexp-alt-list.