Making alloca more safe

[dsimcha]

== Quote from Andrei Alexandrescu (SeeWebsiteForEmail at erdani.org)'s article
> bearophile wrote:
> > Walter Bright:
> >
> >> A person using alloca is expecting stack allocation, and
> >> that it goes away after the function exits. Switching arbitrarily to the
> >> gc will not be detected and may hide a programming error (asking for a
> >> gigantic piece of memory is not anticipated for alloca, and could be
> >> caused by an overflow or logic error in calculating its size).
> >
> > There's another solution, that I'd like to see more often used in Phobos: you
can add another function to Phobos, let's call it salloca (safe alloca) that does
what Denis Koroskin asks for (it's a very simple function).
> Can't be written. Try it.
> Andrei

As a side note, my TempAlloc allocator was intended all along to be a safer and
more flexible allocation scheme that was almost as efficient as call stack
allocation, and does fall back on heap allocation, or creating a new
non-contiguous chunk, when it runs out of space.  Also, I think I'll be able to
fix the GC scanning issue  by fiddling with pointer offset info if/when my precise
heap scanning patch gets into druntime.  If/when TempAlloc can be made both safe
and efficient w.r.t. GC scanning, I'd nominate it for inclusion in Phobos.