In EConstr, defining some "cast" functions earlier.
In EConstr, defining some "cast" functions earlier.

This allows to use a cast in subst_of_rel_context_instance.

Also added more cast functions for further use.

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