bug#70784: Abolish string resizing

[Drew Adams via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Is it only I who am tired of these proposals for complete upheavals
> that, somehow, Emacs has fared just fine without, for generations past?

You're not the only one.

> Backwards-compatibility is an obligation that cannot be
> evaded by means of warnings, which instead serve to annoy
> and antagonize users, whose only wish is that Emacs leave
> them in peace.

Happy to hear that POV.
___

Wrt mutable strings: Consider that Common Lisp, whose
implementations can be very performant, has always had
mutable strings.  I'd even wager that no one ever argued
that its strings shouldn't be mutable.  Inconceivable.

Yes, CL strings are vectors (thus arrays, thus sequences)
of chars, so they're different from Elisp strings.  As
such, you can use sequence functions on them (including
destructive functions such as nreverse).

On the other hand, Elisp strings can have text properties.
(Let's please not hear a proposal to remove that feature
as well, in the name of immutability or performance.)