Avoid resizing mutation in subst-char-in-string, take two

Avoid resizing mutation in subst-char-in-string, take two

This time we take care to preserve properties, and add a test.

* lisp/subr.el (subst-char-in-string):
Use string-replace to avoid resizing mutation and O(n^2) time.
* test/lisp/subr-tests.el (subr--subst-char-in-string): New test.