Update coercion and as-casting rules #2150
Description
In #1951 I noted that the new rule expr.as.pointer.unsized.trait appears to apply to coercions, too. I believe our organization is that all coercion rules should be in the coercion chapter, and the as casting rules just list what additional capabilities as has in addition to the coercion rules. That means I think expr.as.pointer.unsized.trait should be moved to the coercion chapter. While doing this, I think we should review the rules to see what others need updating.
cc @Darksonn