Add some `pure` and `side-effect-free` declarations

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.