Add exp2() as a packet op and array method.

What does this implement/fix?

This exposes exp2(x) as a packet op and coefficient-wise array method.

The generic implementation computes exp2(x) as exp(ln(2)*x) but represents the product ln(2)*x) exactly via the TwoProd algorithm, which significantly reduces the worst case error.

type	max error	max error w/o TwoProd
float	4 ulps	35 ulps
double	110 ulps	363 ulps