allow definition of primary keys directly on kallax.Model tags

[erizocosmico]

Fixes #186

Changes introduced

• Embedded fields now are directly inlined during the processing instead of delegating that to the generation. That's because of technical reasons related to this PR.
• PK can be defined now in the kallax.Model struct tags.
• Other minor changes I came across when developing this feature.

Example usage

type User struct {
  kallax.Model `table:"users" pk:"id,autoincr"`
  ID int64
  Name string
}

Caveats

pk struct tags could be removed from regular fields and keep it all in the model. We could mark as deprecated the previous behavior and remove it on kallax v2.

Docs

Follow-up PR with docs will be made once this is merged, in case something changes during the review.

[codecov]

Codecov Report

Merging #187 into master will increase coverage by 0.21%.
The diff coverage is 95.58%.

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
+ Coverage   78.95%   79.17%   +0.21%     
==========================================
  Files          19       19              
  Lines        3592     3644      +52     
==========================================
+ Hits         2836     2885      +49     
- Misses        544      545       +1     
- Partials      212      214       +2

Impacted Files	Coverage Δ
generator/processor.go	88.73% <100%> (ø)	⬆️
generator/template.go	92.81% <100%> (-0.02%)	⬇️
generator/types.go	86.93% <95.38%> (+0.94%)	⬆️

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