ENH: speed up the semicircular distribution's ppf

Mostly an experiment to try using interpolation in place of a generic calculation.
Timings with master:
```
In [3]: q = np.linspace(0, 1, 100)

In [5]: %timeit stats.semicircular.ppf(q)
10 loops, best of 3: 75.6 ms per loop

In [6]: %timeit stats.semicircular.rvs(size=1000)
1 loops, best of 3: 815 ms per loop
``` 

with this PR:
```
In [35]: %timeit stats.semicircular.ppf(q)
1000 loops, best of 3: 230 µs per loop

In [36]: %timeit stats.semicircular.rvs(size=1000)
10000 loops, best of 3: 155 µs per loop

```