[ssr] Refactor under's Setoid generalization to ease stdlib2 porting

Changes:
* Add ssrclasses.v that redefines [Reflexive] and [iff_Reflexive];
* Add ssrsetoid.v that links
  [ssrclasses.Reflexive] and [RelationClasses.Reflexive];
* Add [Require Coq.ssr.ssrsetoid] in Setoid.v;
* Update ssrfwd.ml accordingly, using a helper file ssrclasses.ml that
  ports some non-exported material from rewrite.ml;
* Some upside in passing: ssrfwd.ml no more depends on Ltac_plugin;
* Update doc and tests as well.

Summary:
* We can now use the under tactic in two flavors:
 - with the [eq] or [iff] relations: [Require Import ssreflect.]
 - or a [RewriteRelation]: [Require Import ssreflect. Require Setoid.]
   (while [ssreflect] does not require [RelationClasses] nor [Setoid],
   and conversely [Setoid] does not require [ssreflect]).
* The file ssrsetoid.v could be skipped when porting under to stdlib2.

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