[lemmas] [proof] Split proof kinds into per-layer components.

We split `{goal,declaration,assumption}_kind` into their
components. This makes sense as each part of this triple is handled by
a different layer, namely:

- `polymorphic` status: necessary for the lower engine layers;
- `locality`: only used in `vernac` top-level constants
- `kind`: merely used for cosmetic purposes [could indeed be removed /
  pushed upwards]

We also profit from this refactoring to add some named parameters to
the top-level definition API which is quite parameter-hungry.

More refactoring is possible and will come in further commits, in
particular this is a step towards unifying the definition / lemma save path.

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