Poor performance of public_address() and public_view_key()
I'm using your project (good work by the way!) to generate hundreds/thousands of addresses and get their public address and private keys. But their generation is, unexpectedly, very slow. Some basic tests in python using [timeit](https://ipython.readthedocs.io/en/stable/interactive/magics.html#magic-timeit) on Intel Core i7-3820 @ 3.8GHz:

```python
In [1]: from monero import seed

In [2]: s = seed.Seed()

In [3]: %timeit s.public_address()
1.08 s ± 9.99 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [4]: %timeit s.public_view_key()
544 ms ± 9.18 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [5]: %timeit s.public_spend_key()
540 ms ± 3.53 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
```

Any reason for this poor performance and any way/fix to make it faster? Generation of thousands addresses and keys on regular basis with that speed (basically one  every 1 to 2 seconds) is slow. 
