save/update inverses before record

[erizocosmico]

Fixes #117

Before, inverses were being treated as any other relationship, thus being saved/updated after the model. If the model had any inverses, that had not been previously saved, the operation would fail, as the inverse is not saved but required to exist in the database.
Now, inverses are saved first, then the model, and, finally, the other relationships.

[codecov]

Codecov Report

Merging #126 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   80.13%   80.24%   +0.11%     
==========================================
  Files          15       15              
  Lines        2743     2759      +16     
==========================================
+ Hits         2198     2214      +16     
  Misses        374      374              
  Partials      171      171

Impacted Files	Coverage Δ
generator/types.go	86.82% <100%> (+0.59%)	⬆️

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[dpordomingo]

lgtm

[smola]

As far as I understood, relationship updates are not atomic. Maybe the documentation should clarify that, so that people is aware that they might need to use a transaction even for a single Save.

[erizocosmico]

@smola the transaction is automatically done by kallax if it has to save more than one record (the case of relationships, for example)