New ecpp primality check in GP

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

Dear PARI developers,

I have merged the implementation by Jared of the ECPP primality
check.

There are three new GP functions:
1) C=ecpp(N): return a primality certificate for N
2) ecppisvalid(C): check if C is a valid ecpp certificate
3) ecppexport(C,2) export C to magma or primo certificate format.

The implementation should be efficient up to 5000 bits.
Various improvement are planned.

Thanks Jared!
Bill.