Accuracy of sinh and complex cos/sin

The `sin` and `cos` function for complex arguments, and the `sinh` function for real arguments, are inaccurate when the inputs are very small. This is because `Math.exp(x) - Math.exp(-x)` returns zero for small x, instead of the more accurate `2x`.

This patch replaces `sinh` by a Taylor expansion when the input is small, which increases accuracy.
I think you should merge this comment into the test description, or at least be more explicit. Notice how in line 33 it explicitly states `(downgrades to number)`, I'm mean something along those lines.