Add more function declarations

* lisp/subr.el (buffer-narrowed-p, sha1, match-substitute-replacement)
(version-to-list, version<, version<=, version=)
(function-get, subregexp-context-p, split-string)
(combine-and-quote-strings, split-string-and-unquote)
(replace-regexp-in-string, syntax-after)
(string-trim-left, string-trim):
* lisp/emacs-lisp/subr-x.el (hash-table-empty-p, hash-table-keys)
(hash-table-values, string-glyph-split)
(string-clean-whitespace, string-fill, string-limit)
(string-pixel-width):
* lisp/env.el (substitute-env-vars, substitute-env-in-file-name)
(setenv-internal):
* lisp/emacs-lisp/rx.el (rx-to-string):
* lisp/emacs-lisp/regexp-opt.el (regexp-opt-depth)
(regexp-opt-charset):
Add appropriate declarations: pure, side-effect-free, and/or
important-return-value.

lisp/emacs-lisp/rx.el

@@ -1144,6 +1144,7 @@ If NO-GROUP is non-nil, don't bracket the result in a non-capturing
 group.
 
 For extending the `rx' notation in FORM, use `rx-define' or `rx-let-eval'."
+  (declare (important-return-value t))
   (let* ((item (rx--translate form))
          (exprs (if no-group
                     (car item)