fix how relationships are saved

[erizocosmico]

Fixes #211

There has been a major change here:

• Recursive saves (even with circular dependencies) are now supported!
• Model interface has changed, but it's not considered public API, so will not trigger a major version bump.

And also a bug has been fixed as a result:

• If the virtual column corresponding to a relationship foreign key is empty (because, for example, the relationship has not been retrieved in the query) but exists in the database, now it is ignored. Before it was deleted without checking if it existed in the database. This does not break any expected behavior because relationships should not be removed by just removing them from the object but using the Remove* methods from the store. All this behavior is now heavily tested.

Thanks to @roobre for reporting this and adding the reproduction case.

[codecov]

Codecov Report

Merging #214 into master will decrease coverage by 0.12%.
The diff coverage is 68.42%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
- Coverage   79.27%   79.14%   -0.13%     
==========================================
  Files          19       19              
  Lines        3628     3635       +7     
==========================================
+ Hits         2876     2877       +1     
- Misses        543      548       +5     
- Partials      209      210       +1

Impacted Files	Coverage Δ
store.go	87.26% <100%> (ø)	⬆️
generator/template.go	92.78% <100%> (-0.04%)	⬇️
model.go	57.05% <60%> (-1.46%)	⬇️

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