This CL adds the implementation for the complete Beta and Lbeta
special functions.
Follow the lead of C++17's std::beta(x,y) and cephes.
This CL investigated 2 different implementations, with the following
performances:
BenchmarkBeta-4 10000000 221 ns/op
BenchmarkBeta2-4 10000000 157 ns/op
Beta uses a slower implementation, using:
exp(lgamma(x)+lgamma(y)-lgamma(x+y))
instead of the faster:
Γ(a)Γ(b) / Γ(a+b)
The former being numerically more stable at extreme values.
