Equations 1.2.2 for Coq 8.12

This is a bugfix release of version 1.2 working with Coq 8.12 (beta1 for now).
The bug fixes in the depind and depelim are potential sources of incompatibility.
See https://mattam82.github.io/Coq-Equations/equations/2019/05/17/1.2.html for the 1.2 release notes.
New in this version:

• Fixed issue #286 : allow let-ins in constructors
• Fixed issue #246: derive subterm for inductives with non-uniform parameters. Also force the user to derive NoConfusion before Subterm, as required for the well-foundedness proof of the Subterm relation. Report an error when the user tries to derive the subterm relation on propositional inductive types (this is impossible).
• PR #285: depelim and noconf were not working up-to simplification of equations. Potential source of incompatibility
• PR #283: depelim and depind no longer simplify unrelated equations in the goal (this was making some subgoals unprovable in some cases). Potential source of incompatibility
• PR #281: add an example of the inspect pattern in the documentation (by Yves Bertot).
• Fixed issue #256: derive eliminators of well-founded definitions in presence of lets in the body.
• Fixed issue #273: derivation of the eliminator was launched too eagerly depending on the order the obligations of a definition were solved.
• Fixed deprecations in OCaml and .v files w.r.t. Coq 8.12