PERF: Use faster version of list comprehension

Using index lookups is ~20% slower (and less pythonic) than unpacking with
comprehension (there's a bit of variability). Plus it makes it more
readable too.

Snippet:

```python
In [1]: def index_unpack(): return [(s[0], s[1]) for s in lst]

In [2]: def pythonic_unpack(): return [(a, b) for a, b in lst]

In [3]: lst = zip(range(100000), range(100000))

In [10]: %timeit -n 100 index_unpack()
100 loops, best of 3: 26.2 ms per loop

In [12]: %timeit -n 100 pythonic_unpack()
100 loops, best of 3: 22 ms per loop
```