Extend the theory of positive rationals `Qp`

See merge request !188

Moreover, and suitable lemmas.

This follows the standard naming for Coq numbers, and avoids one using
the old lemma to break abstraction accidentally.

- Remove the coercion from `Qp` to `Qc`, and remove it into from `Qp_car` into
  `Qp_to_Qc` to be consistent with other conversion functions.
- Use `let '(..) = ...` in the definitions of `Qp_plus`/`Qp_mult`/`Qp_div` to
  avoid Coq tactics (like `injection`) to unfold these definitions eagerly.
- Define orders `Qp_le` and `Qp_lt`, instead of relying on the orders on `Qc`,
  which were obtained through the removed coercion into `Qc`.
- Lift lemmas about the orders from `Qc` to `Qp`.
- Improve variable names and use of notation scopes.

add a function to obtain a set with all elements of a finite type

See merge request !196

add dom_insert_lookup

See merge request !195

Remove dead type classes and notations.

See merge request !197

These were very specific, there were no lemmas about them. They were used
back in the days for some specific things in my C semantics.

Add map_fmap_union for FinMap

See merge request !194

Avoid relying on implicit instance generalization, and name some instances.

See merge request !193

Fix in preparation for https://github.com/coq/coq/pull/13188

Drop support for Coq 8.8 and 8.9

See merge request !190