bug#31750: simplify and tune Emacs stack-related allocation

[Paul Eggert]

Eli Zaretskii wrote:
> OTOH, maybe I do agree, as long as "blindly" is an essential part of
> that rule ;-)

Yes, the "blindly" is essential. It's OK to use xmint_ptr on any mint_ptr, so 
long as you don't blindly trust the result to be a valid C pointer of the kind 
that you want; that is, you have to know that your C code created the mint_ptr 
in question and that no Lisp code can have replaced the mint_ptr with some other 
object (perhaps also a mint_ptr). This is like XSAVE_POINTER, where you also 
have to know that Lisp code has not replaced the Lisp_Save_Value object with 
some other object (perhaps also of type Lisp_Save_Value and with a pointer payload).

The main difference is that Lisp code can easily coin a mint_ptr (simply by 
using a fixnum) whereas it can't easily coin a Lisp_Save_Value, so there's less 
runtime checking to catch bugs in the C code. However, if two or more 
Lisp_Save_Value objects are exposed to Lisp code then the Lisp code can pass one 
where the other is expected and this can cause the same sort of crash as passing 
a fixnum where a mint_ptr is expected, so introducing mint_ptr doesn't make 
things that much more dangerous in principle than they were before.