Variadic function for multiplications with mpz_t

[Marc Glisse]

On Mon, 12 Jan 2015, Ha T. Lam wrote:

> ​Unfortunately, there are a lot of GMP functions that are not available
> with ​the wrapper class, e.g. invert, gcd, nextprime, (non-negative) mod,
> etc.

At least gcd is available (I added it in June according to mercurial). My 
next goal is factorial. More functions will slowly come.

-- 
Marc Glisse