Improving primecert(n, 0)

[<bobbaillie@frii.com>]

The primecert(n, 0) function generates prime certificates (prime proofs)
and is very useful!

However, I have noticed two limitations, at least on Windows:
1) it uses only one core no matter how many the machine has.
2) the amount of memory it needs seems to increase as the square of the
number of digits.
Empirically: if T is the number of digits in Thousands, the number of
gigabytes of RAM needed will be about T^2. So, a 7000 digit number would
need about 49 GB of RAM.

The Primo program (linux only) also produces prime certificates. Primo
uses all cores (if requested), and only one or two gigabytes of RAM, even
for 12,000 digit numbers.

Can pari's primecert function be improved along these lines?