Fix bug #5435: [Eval native_compute in] raises anomaly.
Fix bug #5435: [Eval native_compute in] raises anomaly.

Was introduced by 4f041384cb27f0d2. Unsoundness seems miraculously
avoided by a safeguard I put in nativecode.ml. But other kernel changes
in this commit should probably be reviewed carefully.

SkySkimmer/coq,silene/coq,coq/coq,Lionel-Rieg/coq,gares/coq,Zimmi48/coq,maximedenes/coq,Zimmi48/coq,SkySkimmer/coq,JasonGross/coq,william-lawvere/coq,Zimmi48/coq,william-lawvere/coq,coq/coq,gares/coq,amintimany/coq,letouzey/coq-wip,ppedrot/coq,gares/coq,Matafou/coq,maximedenes/coq,ppedrot/coq,letouzey/coq-wip,Lionel-Rieg/coq,ejgallego/coq,Matafou/coq,Zimmi48/coq,amintimany/coq,SkySkimmer/coq,JasonGross/coq,amintimany/coq,Lionel-Rieg/coq,maximedenes/coq,ppedrot/coq,ppedrot/coq,ejgallego/coq,ejgallego/coq,Matafou/coq,SkySkimmer/coq,JasonGross/coq,JasonGross/coq,maximedenes/coq,letouzey/coq-wip,william-lawvere/coq,letouzey/coq-wip,maximedenes/coq,silene/coq,coq/coq,coq/coq,gares/coq,ejgallego/coq,silene/coq,ppedrot/coq,letouzey/coq-wip,gares/coq