Frequent items generic support

[jmalkin]

I'm very much torn on whether defining operator<< on py::object is a good idea, even with it static to limit the scope. The sketch's to_string() method won't work without that, though. Even this way, it requires there to be a __str__() method defined for the python class to work usefully.

Unless there's a better way the only alternatives are that we write a new to_string() for generic objects calling the method directly, write a version that can't list the objects, or remove that method entirely.

But this feels rather hacky so I'm open to suggestions.

[jmalkin]

Just realized I have no tests! That's a problem... But in the meantime, feel free to comment on whether this is even a viable approach.

[coveralls]

Pull Request Test Coverage Report for Build 4148899339

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 19 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+3.3%) to 97.141%

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Files with Coverage Reduction	New Missed Lines	%
theta/include/theta_sketch_impl.hpp	6	8.35%
theta/include/compact_theta_sketch_parser_impl.hpp	13	82.98%
<!--	Total:	19		-->

Totals
Change from base Build 4077312869:	3.3%
Covered Lines:	4927
Relevant Lines:	5072

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💛 - Coveralls