translate most functions in erf.jl in Julia's form

[ziyixi]

Translate most functions in erf.jl except for functions with the BigFloat input. The code is directly translated from apple's libm for erf(x::Float64), erfc(x::Float64) and libopenspecfun for the other functions. The licenses of both the libs are MIT so I could directly translate them .

All the functions have passed the tests. And you could see the benchmark in https://github.com/ziyixi/gsoc-work/blob/master/benchmark.ipynb .

[stevengj]

Can you make a plot of the ratios new/old of the minimum (not median) times?

[Ken-B]

@ziyixi Could you update the link to the notebook, it seems dead. Thanks!