more efficient scoring iterations

[ajinkya-k]

Hi @ararslan

I had some ideas on how to speed up the scoring algorithm. Are you alreaady working along these lines? If not, is it okay if I take a crack at it?

[ajinkya-k]

Some fairly minor tweaks got me some decent speedup!

julia> @be fit!(bm)
Benchmark: 2517 samples with 1 evaluation
 min    31.459 μs (242 allocs: 31.844 KiB)
 median 33.042 μs (242 allocs: 31.844 KiB)
 mean   37.471 μs (242 allocs: 31.844 KiB, 0.19% gc time)
 max    2.274 ms (242 allocs: 31.844 KiB, 97.26% gc time)

julia> @be BetaRegression.fit2!(bm)
Benchmark: 3146 samples with 1 evaluation
 min    26.958 μs (172 allocs: 27.406 KiB)
 median 27.584 μs (172 allocs: 27.406 KiB)
 mean   28.967 μs (172 allocs: 27.406 KiB, 0.15% gc time)
 max    1.844 ms (172 allocs: 27.406 KiB, 97.55% gc time)