IsPattern coercions aren't properly implemented since they rely on
doing a runtime cast, which can fail for things like function subtyping.
Make clients choose what behavior they want. Unfortunately we need to
preserve the current behavior in places since it's relied upon for
things like exhaustivity checking. We ought to properly implement
coercion handling, then we should be able to remove this.
Although I don't plan to bring over new assertions wholesale
into the current qualification branch, it's entirely possible
that various minor changes in main will use the new assertions;
having this basic support in the release branch will simplify that.
(This is why I'm adding the includes as a separate pass from
rewriting the individual assertions)
