Add some pure and side-effect-free declarations

* lisp/subr.el (string-to-list, string-to-vector, string-or-null-p)
(booleanp, special-form-p, plistp, macrop, compiled-function-p)
(flatten-tree):
* lisp/emacs-lisp/subr-x.el (string-join, string-truncate-left)
(string-blank-p, string-remove-prefix, string-remove-suffix)
(string-pad, string-chop-newline):
Declare functions pure, side-effect-free, and/or error-free.

lisp/emacs-lisp/subr-x.el

@@ -102,6 +102,7 @@ threading."
   "Join all STRINGS using SEPARATOR.
 Optional argument SEPARATOR must be a string, a vector, or a list of
 characters; nil stands for the empty string."
+  (declare (pure t) (side-effect-free t))
   (mapconcat #'identity strings separator))
 
 (define-obsolete-function-alias 'string-reverse 'reverse "25.1")
@@ -112,6 +113,7 @@ characters; nil stands for the empty string."
 When truncating, \"...\" is always prepended to the string, so
 the resulting string may be longer than the original if LENGTH is
 3 or smaller."
+  (declare (pure t) (side-effect-free t))
   (let ((strlen (length string)))
     (if (<= strlen length)
 	string
@@ -124,16 +126,19 @@ the resulting string may be longer than the original if LENGTH is
   "Check whether STRING is either empty or only whitespace.
 The following characters count as whitespace here: space, tab, newline and
 carriage return."
+  (declare (pure t) (side-effect-free t))
   (string-match-p "\\`[ \t\n\r]*\\'" string))
 
 (defsubst string-remove-prefix (prefix string)
   "Remove PREFIX from STRING if present."
+  (declare (pure t) (side-effect-free t))
   (if (string-prefix-p prefix string)
       (substring string (length prefix))
     string))
 
 (defsubst string-remove-suffix (suffix string)
   "Remove SUFFIX from STRING if present."
+  (declare (pure t) (side-effect-free t))
   (if (string-suffix-p suffix string)
       (substring string 0 (- (length string) (length suffix)))
     string))
@@ -252,6 +257,7 @@ is done.
 If START is nil (or not present), the padding is done to the end
 of the string, and if non-nil, padding is done to the start of
 the string."
+  (declare (pure t) (side-effect-free t))
   (unless (natnump length)
     (signal 'wrong-type-argument (list 'natnump length)))
   (let ((pad-length (- length (length string))))
@@ -261,6 +267,7 @@ the string."
 
 (defun string-chop-newline (string)
   "Remove the final newline (if any) from STRING."
+  (declare (pure t) (side-effect-free t))
   (string-remove-suffix "\n" string))
 
 (defun replace-region-contents (beg end replace-fn

lisp/subr.el

@@ -4124,15 +4124,18 @@ system's shell."
 
 (defsubst string-to-list (string)
   "Return a list of characters in STRING."
+  (declare (side-effect-free t))
   (append string nil))
 
 (defsubst string-to-vector (string)
   "Return a vector of characters in STRING."
+  (declare (side-effect-free t))
   (vconcat string))
 
 (defun string-or-null-p (object)
   "Return t if OBJECT is a string or nil.
 Otherwise, return nil."
+  (declare (pure t) (side-effect-free error-free))
   (or (stringp object) (null object)))
 
 (defun list-of-strings-p (object)
@@ -4145,21 +4148,25 @@ Otherwise, return nil."
 (defun booleanp (object)
   "Return t if OBJECT is one of the two canonical boolean values: t or nil.
 Otherwise, return nil."
+  (declare (pure t) (side-effect-free error-free))
   (and (memq object '(nil t)) t))
 
 (defun special-form-p (object)
   "Non-nil if and only if OBJECT is a special form."
+  (declare (side-effect-free error-free))
   (if (and (symbolp object) (fboundp object))
       (setq object (indirect-function object)))
   (and (subrp object) (eq (cdr (subr-arity object)) 'unevalled)))
 
 (defun plistp (object)
   "Non-nil if and only if OBJECT is a valid plist."
+  (declare (pure t) (side-effect-free error-free))
   (let ((len (proper-list-p object)))
     (and len (zerop (% len 2)))))
 
 (defun macrop (object)
   "Non-nil if and only if OBJECT is a macro."
+  (declare (side-effect-free t))
   (let ((def (indirect-function object)))
     (when (consp def)
       (or (eq 'macro (car def))
@@ -4169,6 +4176,7 @@ Otherwise, return nil."
   "Return non-nil if OBJECT is a function that has been compiled.
 Does not distinguish between functions implemented in machine code
 or byte-code."
+  (declare (side-effect-free error-free))
   (or (subrp object) (byte-code-function-p object)))
 
 (defun field-at-pos (pos)
@@ -6843,6 +6851,7 @@ returned list are in the same order as in TREE.
 
 \(flatten-tree \\='(1 (2 . 3) nil (4 5 (6)) 7))
 => (1 2 3 4 5 6 7)"
+  (declare (side-effect-free error-free))
   (let (elems)
     (while (consp tree)
       (let ((elem (pop tree)))