Replace Rmath hypergeometric with pure Julia implementation

Port hypergeometric distribution functions from Ian Smith's VBA code,
removing the dependency on Rmath for all 10 hyper functions (pdf,
logpdf, cdf, ccdf, logcdf, logccdf, invcdf, invccdf, invlogcdf,
invlogccdf).

Add helper functions logfbit, lfbaccdif1, and ab_minus_cd to misc.jl
for accurate Stirling correction and cross-product computation.

Author: andreasnoack