Use whd-all on rigid-flex conversion.
Use whd-all on rigid-flex conversion.

This heuristic is justified by the fact that during a conversion check
between a flexible and a rigid term, the flexible one is eventually going
to be fully weak-head normalized. So in this case instead of performing
many small reduction steps on the flexible term, we perform full weak-head
reduction, including delta.

It is slightly more efficient in actual developments, and it fixes a corner
case encountered by Jason Gross.

Fixes #6667: Kernel conversion is much, much slower than `Eval lazy`.

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