faster merge_sorted without key

Before:

>>> from heapq import merge
>>> from toolz import merge_sorted
>>> a = range(10000)
>>> b = range(1, 5000, 2)
>>> %timeit list(merge(a, b))
100 loops, best of 3: 2.02 ms per loop
>>> %timeit list(merge_sorted(a, b))
100 loops, best of 3: 2.43 ms per loop

After:

>>> %timeit list(merge_sorted(a, b))
1000 loops, best of 3: 1.94 ms per loop
>>> %timeit list(merge(a, b))
100 loops, best of 3: 1.91 ms per loop
Merge pull request #163 from larsmans/faster-merge-sorted

faster merge_sorted without key