Add real log1p

[pearu]

Complex log1p and other complex functions relay on real log1p and hence real log1p defines the accuracy properties of these complex functions. Per https://github.com/pearu/functional_algorithms/tree/main/results, the native real log1p has some room for improvement accuracy wise.

Plan: use Remez algorithm to find polynomial approximation for real log1p. See existing implementations in

• https://github.com/DKenefake/OptimalPoly/
• https://go.dev/src/math/log1p.go