Use lexical-binding in a few files where it was easy.
Don't require `proof-compat` when it's not used.

* coq/coq-db.el: Use lexical-binding.

* coq/coq-system.el: Use lexical-binding.
(coq--extract-prog-args): Use concatenated-args rather than recomputing it.

* coq/coq.el: Require `span` to silence some warnings.

* generic/pg-user.el: Use lexical-binding.
(complete, add-completion, completion-min-length): Silence warnings.

* generic/pg-xml.el: Use lexical-binding.
(pg-xml-string-of): Prefer mapconcat to reduce+concat.

* generic/proof-depends.el: Use lexical-binding.
(proof-dep-split-deps): Use `push`.

* generic/proof-shell.el: Require `span` to silence some warnings.
(proof-shell-invisible-command): Don't use lexical-let just to build
a wasteful η-redex!

* lib/holes.el: Use lexical-binding.
Remove redundant :group args.

* lib/span.el: Use lexical-binding.
(span-read-only-hook): Use user-error.
(span-raise): Remove, unused.

This addresses part of the issues pointed out in #112

* coq/coq-indent.el (coq-indent-inner-regexp): Remove old X-Symbol element.
(coq-save-count, coq-proof-count):
* obsolete/plastic/plastic.el (plastic-shell-handle-output):
* lib/texi-docstring-magic.el (texi-docstring-magic-insert-magic):
* lib/pg-dev.el (emacs-lisp-mode-hook):
* lib/maths-menu.el (maths-menu-filter-predicate)
(maths-menu-tokenise-insert):
* lib/holes.el (holes-next):
* lego/lego.el (lego-shell-handle-output):
* isar/isabelle-system.el (isabelle-docs-menu):
* coq/coq.el (coq-compile-command, coq-compile-auto-save)
(coq-compile-ignored-directories, coq-load-path-safep)
(proof-shell-handle-delayed-output-hook): Don't quote lambda.

spurious warning in Emacs 23.2