Remove the return value of String::splice

[dtolnay]

As discussed in #32310, it is never necessary to look at the return value of String::splice. Applying the splice eagerly is always simpler and at least as efficient.

- fn splice<R>(&'a mut self, range: R, replace_with: &'b str) -> Splice<'a, 'b>;
+ fn splice<R>(&mut self, range: R, replace_with: &str);

With a return value:

1. The String and the replacement str are in scope (start of 'a and 'b).
2. Call splice, receive Splice struct.
3. The String is exclusively borrowed and unusable.
4. Process the replaced segment one char at a time.
5. Drop the Splice struct.
6. Drop the String and the replacement str (end of 'a and 'b, must happen after 5).

Without a return value;

1. The String and the replacement str are in scope (start of 'a and 'b).
2. Grab the soon replaced segment &s[range].
3. The String is shared borrowed but otherwise usable.
4. Process the soon replaced segment one char at a time or as an ordinary &str.
5. Call splice which eagerly does the replace and returns ().
6. Drop the String and the replacement str (end of 'a and 'b).