Fast path for Pol.Mul(x, 2)
Fast path for Pol.Mul(x, 2)

Benchmark results on amd64 show a 13-fold speedup:

name              old time/op  new time/op  delta
PolIrreducible-8  57.4ms ± 0%   4.3ms ± 0%  -92.43%  (p=0.000 n=10+7)

The mul2 call could be inlined in Pol.MulMod for a further 7% speedup,
but the tests don't cover that.

Compared to 3404fc8a9294eb080f20d955a013acef55c9de86, we have:

name                old time/op  new time/op  delta
RandomPolynomial-8   102ms ±13%     8ms ± 8%  -92.55%  (p=0.000 n=10+10)
PolIrreducible-8    62.4ms ± 0%   4.3ms ± 0%  -93.04%  (p=0.000 n=10+7)

restic/chunker