Added slices to the prime sieve.

The following syntax is now possible when using the prime sieve:

```python
from sympy import sieve

assert sieve[5] == 11
assert list(sieve[5:10]) == [sieve[x] for x in range(5, 10)]
assert list(sieve[5:10:2]) == [sieve[x] for x in range(5, 10, 2)]
```