Support for primitive projections in tc solver

[Tragicus]

The instance compiler does not like primitive projections:

From elpi Require Import tc.

Set Primitive Projections.
Record S := { sort :> Type }.
Unset Primitive Projections.

Class C (s : Type) := {}.

Fail Instance SC (s : S) : C s := Build_C s.

[gares]

I think @FissoreD and myself are busy until mid April, then we will come back to the TC solver and eventually fix this.

[FissoreD]

Hello Quentin,
I have prepared a rapid patch that seems to solve your problem in (#793).
I haven't investigate too much tough
Hope it solves your pb