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/regexp-opt.el

@@ -154,6 +154,7 @@ usually more efficient than that of a simplified version:
   "Return the depth of REGEXP.
 This means the number of non-shy regexp grouping constructs
 \(parenthesized expressions) in REGEXP."
+  (declare (pure t) (side-effect-free t))
   (save-match-data
     ;; Hack to signal an error if REGEXP does not have balanced parentheses.
     (string-match regexp "")
@@ -270,6 +271,7 @@ Merges keywords to avoid backtracking in Emacs's regexp matcher."
 CHARS should be a list of characters.
 If CHARS is the empty list, the return value is a regexp that
 never matches anything."
+  (declare (pure t) (side-effect-free t))
   ;; The basic idea is to find character ranges.  Also we take care in the
   ;; position of character set meta characters in the character set regexp.
   ;;

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)

lisp/emacs-lisp/subr-x.el

@@ -81,18 +81,22 @@ Note how the single `-' got converted into a list before
 threading."
   (declare (indent 0) (debug thread-first))
   `(internal--thread-argument nil ,@forms))
+
 (defsubst hash-table-empty-p (hash-table)
   "Check whether HASH-TABLE is empty (has 0 elements)."
+  (declare (side-effect-free t))
   (zerop (hash-table-count hash-table)))
 
 (defsubst hash-table-keys (hash-table)
   "Return a list of keys in HASH-TABLE."
+  (declare (side-effect-free t))
   (let ((keys nil))
     (maphash (lambda (k _) (push k keys)) hash-table)
     keys))
 
 (defsubst hash-table-values (hash-table)
   "Return a list of values in HASH-TABLE."
+  (declare (side-effect-free t))
   (let ((values nil))
     (maphash (lambda (_ v) (push v values)) hash-table)
     values))
@@ -149,6 +153,7 @@ carriage return."
 All sequences of whitespaces in STRING are collapsed into a
 single space character, and leading/trailing whitespace is
 removed."
+  (declare (important-return-value t))
   (let ((blank "[[:blank:]\r\n]+"))
     (string-trim (replace-regexp-in-string blank " " string t t)
                  blank blank)))
@@ -158,6 +163,7 @@ removed."
 Wrapping is done where there is whitespace.  If there are
 individual words in STRING that are longer than LENGTH, the
 result will have lines that are longer than LENGTH."
+  (declare (important-return-value t))
   (with-temp-buffer
     (insert string)
     (goto-char (point-min))
@@ -189,6 +195,7 @@ coding system that doesn't specify a BOM, like `utf-16le' or `utf-16be'.
 When shortening strings for display purposes,
 `truncate-string-to-width' is almost always a better alternative
 than this function."
+  (declare (important-return-value t))
   (unless (natnump length)
     (signal 'wrong-type-argument (list 'natnump length)))
   (if coding-system
@@ -324,6 +331,7 @@ as the new values of the bound variables in the recursive invocation."
 ;;;###autoload
 (defun string-pixel-width (string)
   "Return the width of STRING in pixels."
+  (declare (important-return-value t))
   (if (zerop (length string))
       0
     ;; Keeping a work buffer around is more efficient than creating a
@@ -344,6 +352,7 @@ This takes into account combining characters and grapheme clusters:
 if compositions are enabled, each sequence of characters composed
 on display into a single grapheme cluster is treated as a single
 indivisible unit."
+  (declare (side-effect-free t))
   (let ((result nil)
         (start 0)
         comp)