bug#70784: Abolish string resizing

[Mattias Engdegård]

To minimise even low-probability trouble, I've looked for standard Elisp 
functions that might run into the resizing mutation trouble if used in a 
specific way. There's not a lot, but so far I've done:

- cl-substitute[-if[-not]]
- cl-sort (not that sorting strings is very common)
- subst-char-in-string with INPLACE = nil

I'm going to leave functions that explicitly mutate strings alone since code 
may depend on them doing that which is completely fine.

The manual would benefit from some light editing, just to make it clear that 
mutating multibyte strings is not usually the best programming style. I'll post 
a suggested patch.

Also, I'm leaning toward removing `store-substring` from the manual because 
while it probably felt like a good addition at the time, circumstances have 
changed and it has hardly seen any use at all. (We can mark it obsolete or keep 
it indefinitely but it doesn't really deserve top billing these days.)