performance improvements to _factorize_str_helper

[ARF1]

Speedup ca. x1.5 cf. master branch on my machine with compressed bcolz Approximate contributions:

• 1/3: direct indexing into arrays using typed memoryviews
• 1/3: substitution of reverse dict with std::vector objects
• 1/3: use of nested with nogil, with gil construct

[ARF1]

Uncompressed bcolz timings on my machine:

bquery master:
In [3]: %timeit -r 10 a.cache_factor(['mycol'], refresh=True)
1 loops, best of 10: 2.46 s per loop

pull request:
In [3]: %timeit -r 10 a.cache_factor(['isin'], refresh=True)
1 loops, best of 10: 1.59 s per loop

==> Factor: 1.5

Compressed bcolz timings on my machine:

bquery master:
In [3]: %timeit -r 10 a.cache_factor(['mycol'], refresh=True)
1 loops, best of 10: 4.03 s per loop

pull request:
In [3]: %timeit -r 10 a.cache_factor(['mycol'], refresh=True)
1 loops, best of 10: 3.13 s per loop

==> Factor: 1.3

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Possible additional optimizations (but probably fairly minor):

• There should be no need to store reverse_keys in _factorize_str_helper since it is merely an increasing sequence of integers up to reverse_values.size-1 (I wanted to keep the code logic as close to the original as possible.)
• In factorize_str the reverse python dictionary (insertion expensive hash-table, I think) is created only to throw it away after creation of carray_values (Changing this would have obvious knock-on effects on other helper functions.)