Use emplace instead of insert/push in ObserverPool::add

[frmdstryr]

This avoids an extra xincref/xdecref on the cppy::ptr because insert & push_back do a copy.

See https://godbolt.org/z/zGvTzGahn

The *obs_free = Observer() does the same thing but I'm not sure how to cleany fix that (other than manually setting the fields).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.67%. Comparing base (0750392) to head (c666579).
Report is 2 commits behind head on main.

Additional details and impacted files

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##             main     #242   +/-   ##
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  Coverage   97.67%   97.67%           
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  Files          24       24           
  Lines        1074     1074           
  Branches      162      162           
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  Hits         1049     1049           
  Misses         12       12           
  Partials       13       13           

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[sccolbert]

For the last part, I think you can do:

obs_free->m_change_types = change_types;
obs_free->m_observer = observer;

Or you could add a method to the Observer struct to swap the fields for you. You could add a couple overloads to that method to also eliminate the other places where we manually twiddle the field.

Honestly, I can't remember why I made Observer a struct instead of class with more encapsulated behavior.

I think I would take the later approach if I were to do it again.