Simplify interleave.

[groutr]

Simplified the implementation of interleave and made it slightly faster.

seqs = [range(1000) for i in range(100)]
%timeit list(interleave(seqs))

old: 5.07 ms ± 240 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
new: 4.03 ms ± 45.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

[eriknw]

Thanks for working to clean this up and make it faster @groutr!

I noticed a difference with the new implementation:

>>> import toolz, cytoolz
>>> list(toolz.interleave([[1, 2], 'abcdef']))
[1, 'a', 2, 'b', 'c', 'd', 'e', 'f']
>>> list(cytoolz.interleave([[1, 2], 'abcdef']))
[1, 'a', 2, 'b', 'c']