binomial no longer uses Float64 internally.

Replaces the old accumulation step, using x=*(nn::Float64, rr::Float64)
with x::T=xt=div(widemul(x,nn), rr). Also tests for integer overflow.

Fix #6154.
Merge pull request #6163 from JuliaLang/cjh/fix-6154

binomial no longer uses Float64 internally.