Fixing one part of #5669 (unification heuristics sensitive to choice of names).
Fixing one part of #5669 (unification heuristics sensitive to choice of names).

This surprising bug was caused by an Id.Set which was ordering
solutions to variable-projection problems in ascii order.

We fix it by re-considering the variables involved in the solutions
using the declaration order.

Note that in practice, this implies preferring a dependent solution
over a non-dependent one.

coq/coq,gares/coq,gares/coq,silene/coq,Zimmi48/coq,maximedenes/coq,letouzey/coq-wip,coq/coq,JasonGross/coq,coq/coq,ejgallego/coq,gares/coq,Zimmi48/coq,Matafou/coq,ejgallego/coq,Matafou/coq,maximedenes/coq,ppedrot/coq,Matafou/coq,JasonGross/coq,JasonGross/coq,letouzey/coq-wip,gares/coq,coq/coq,SkySkimmer/coq,SkySkimmer/coq,ejgallego/coq,JasonGross/coq,silene/coq,SkySkimmer/coq,letouzey/coq-wip,ejgallego/coq,letouzey/coq-wip,maximedenes/coq,ppedrot/coq,ppedrot/coq,letouzey/coq-wip,maximedenes/coq,gares/coq,maximedenes/coq,SkySkimmer/coq,ppedrot/coq,Zimmi48/coq,Zimmi48/coq,ppedrot/coq,silene/coq