Add python serde support for varopt sketches

[jmalkin]

The immediate result is for varopt, but having this working should be an important part of adding tuple support.

I'm least confident in the error recovery during deserialization. I think it's most likely that Python will throw an error which propagates through the c++ code. In this case I'm intercepting it to try to decrement reference counts for items we've already allocated but it's hard to tell if that's actually preventing a leak or not. Happy for suggestions on how to clean up that code.

Also, fixed a size bug in varopt.

[coveralls]

Pull Request Test Coverage Report for Build 2752291174

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+5.9%) to 98.227%

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Totals
Change from base Build 2713877899:	5.9%
Covered Lines:	9420
Relevant Lines:	9590

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💛 - Coveralls

[AlexanderSaydakov]

I don't think I completely understand how all this comes together, but looks fine to me