This adds an expm1 function for complex values. It uses the property that

cos(x) - 1 = -2*sin(x/2)^2

There is some cancellation error when real(z) ~ imag(z)^2/2 for small z, but otherwise is fairly stable.
add more tests
Merge pull request #6539 from simonbyrne/expm1

Adds an expm1 function for complex values.