bug#43598: replace-in-string: finishing touches

[Mattias Engdegård]

The new replace-in-string function is welcome but needs a few tweaks before we 
can call it done:

1. It doesn't quite work correctly with raw bytes:

  (replace-in-string "\377" "x" "a\377b")
  => "axb"
  (replace-in-string "\377" "x" "a\377ø")
  => "a\377ø"

The easiest solution is to reimplement it in terms of replace-regexp-in-string 
for now, and optimise it later (although I feel a bit bad undoing Lars's pretty 
handiwork...)

We have messy semantics here, because string-equal does not equate "\377" and 
(string-to-multibyte "\377"), but string-match-p does...

2. It is documented always to return a new string, but that's a tad 
over-generous nowadays; very few string functions do that. If we drop that 
guarantee, we get some optimisation opportunities:

- it can return the input string itself if no matches were found (a fairly 
common case)
- it can be marked pure, not just side-effect-free, so that the byte compiler 
can constant-propagate through calls to it

3. The name is somewhat unfortunate since a function by that name in XEmacs 
uses regexp matching.
In fact, the new function probably broke prolog-mode because of that (see 
prolog-replace-in-string).
While we can fix prolog-mode, we can't easily fix code outside the Emacs tree 
that may have similar problems.

Perhaps we should rename it to string-replace, in line with the modern naming 
convention discussed some time ago.