bug#18410: Use SAFE_ALLOCA etc. to avoid unbounded stack allocation.

[Paul Eggert <eggert@cs.ucla.edu>]

Dmitry Antipov wrote:
> USE_SAFE_ALLOCA;                                                    |-
> ptrdiff_t count = SPECPDL_INDEX ();                    |-           |
> ...                                                    | inner bind |
> outer bind
> Lisp_Object result = unbind_to (count, Fsome_func ()); |-           |
> SAFE_FREE ();                                                       |-
> return result;
>
> looks suboptimal because it calls unbind_to twice.

I noticed that too, and actually coded up something along those lines, 
but decided to discard it as it added complexity and the patch was 
already pretty large.  There is some virtue in having a simpler API, 
even if it's a tad suboptimal.  Perhaps we can think of a way of 
combining SAFE_FREE and unbind_to so that there aren't two different 
ways in the source code of doing the same thing.

To be honest I've never been a fan of 'RETURN_UNGCPRO (expr);', and 
would rather not encourage other macros along those lines.  I was hoping 
that we could get rid of all the GCPRO stuff, and simplify the code 
under the assumption that GC_MARK_STACK == GC_MAKE_GCPROS_NOOPS.

I do like the idea about specbind returning the previous SPECPDL_INDEX, 
as that would simplify the code.