leftright_pow and sliding window powering algorithm

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

Dear PARI developers,

I have renamed leftright_pow to gen_pow and leftright_pow_u to gen_powu,
and I have implemented the sliding window powering algorithm, which is
often 30% faster, when multiplications are slow.

Code that depended on the undocumented fact that leftright_pow only ever
multiplied by the basis should be fixed to use
leftright_pow_fold/leftright_pow_u_fold.

Cheers,
Bill.