ENH: special: faster/more precise expit (logistic sigmoid)

Reducing the positive case to tanh gives better precision:

>>> x = 37
>>> 1 / (1 + exp(-x))
1.0
>>> 0.5*(1 + tanh(0.5*x))
0.99999999999999989

It's also a few percent faster to compute (on x86-64 w/ GCC and Glibc).