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

[Lars Ingebrigtsen]

Mattias Engdegård <mattiase@acm.org> writes:

> 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"

I went ahead and checked in a new C-level function string-search, which
should be an efficient way to search for strings in strings (using
memmem, which Emacs has via Gnulib?), and this fixed these corner cases.

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