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.
Yes, there is a round-off error here. Maybe replace the added `assert.deepEqual(...)` with

```js
approx.deepEqual(cos(complex('1e-50+1e-50i')), complex(1, -1e-100));
```