Implement BBHashTable in Cython and optimize lookup loops.

[ctb]

Implement a bbhash lookup table in Cython.

This permits queries by hash to retrieve values, with hashes that may not be in the MPHF index.

Full implementation here, optimized lookup function is here, and some simple test code is here.

Simple benchmark here.

A few thoughts on further optimizations to BBHashTable.get_unique_values:

• the hashes argument is of unknown length, so can't easily be required to be a numpy array. I think the current approach works pretty well if it is a numpy array, although it probably copies it unnecessarily.
• the self.mphf.lookup could be done entirely in C/Cython land quite easily, I think. Just need to produce headers that let bbhash_table.pyx know details about bbhash.pyx.
• can we switch to doing something clever with STL maps instead of defaultdict?

Other thoughts --

• [x] add more tests!
• [ ] can we conda-ize this?

[ctb]

• XX test different size data types as values (removed this functionality for now)
• [x] test/show off for loop vs get_unique_values in tests

[ctb]

• [x] move to github actions

[codecov[bot]]

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[ctb]

• [x] update BooPHF

[ctb]

I'm going to declare victory and merge this, for now. Real-world performance is sufficiently good that further updates can happen in new versions!