branch bill-pari_mainstack

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear PARI developers,

I have created a branch bill-pari_mainstack.
In this branch, we use the same function for handling the PARI stack
and the threads substacks (if any).

There is now a global pointer pari_mainstack  which point to the current
mainstack. As a consequence, top is renamed to pari_mainstack->top and bot to
pari_mainstack->bot.

Instead of pari_init_stack and allocatemem, there is now paristack_alloc and
paristack_resize (which are thin wrapper around pari_mainstack_*).

Cheers,
Bill.