Make 'separate_bodies' default to false, both in Rust and in JSON

[adamchalmers]

If you put #[serde(default)] it'll use the default value for that field's type (e.g. ints default to 0, strings default to "", and you can define whatever defaults you want for your type.

If you put #[builder(default)] then the builder API will use the default too, and users can override that default of course.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.19%. Comparing base (754c322) to head (f35fa4a).
⚠️ Report is 1 commits behind head on mike/csg-separate-bodies.

Additional details and impacted files

@@                    Coverage Diff                    @@
##           mike/csg-separate-bodies    #1089   +/-   ##
=========================================================
  Coverage                     26.19%   26.19%           
=========================================================
  Files                            35       35           
  Lines                          1756     1756           
=========================================================
  Hits                            460      460           
  Misses                         1296     1296           

Flag	Coverage Δ
unittests	26.19% <ø> (ø)

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