Fix asinh of negative values

When `x` has large magnitude, `x + ((x * x) + 1.0).sqrt()` approaches
`x + x.abs()`. For negative values of `x`, this leads to catastrophic
cancellation, resulting in large errors or even 0 being passed to
`ln`, producing incorrect results including `-inf`.

Becuase asinh is an odd function, i.e. -asinh(x) = asinh(-x) for all
x, we can avoid the catastrophic cancellation and obtain correct
results by taking the absolute value of `self` for the first
term. `self * self` is always positive, so in effect this gives us
`x.abs().asinh().copysign(x)` which as discussed above is
algebraically equivalent, but is much more accurate.

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