`__name__` attribute of `exp`, `sin`, `cos`, etc is `'f'`

The `__name__` attribute of several basic function is `'f'`, when presumably it should be the actual function name.

For example,

```
In [1]: import mpmath

In [2]: mpmath.__version__
Out[2]: '1.4.0b1'

In [3]: from mpmath import mp

In [4]: mp.exp.__name__
Out[4]: 'f'

In [5]: mp.sin.__name__
Out[5]: 'f'

In [6]: mp.tan.__name__
Out[6]: 'f'
```
