Fix `exp(-inf)` being negative infinity

[benjamin-cates]

exp(neg inf) should actually be zero.

Is this the proper way to create an instance of zero (like a global const or something). I just copied how I saw 1 being created somewhere else in the file.

[benjamin-cates]

Oh I now see that some other boundary conditions are written as BigFloat::new(1). Is this a better way to construct it or is it slower?

[stencillogic]

Thanks for pointing to this. Good catch!

[stencillogic]

Closing as done.