Feature: disallow type guards

[billwanjohi]

Type guards are a more systematic way of asserting a type, but they are still prone to unsoundness, particularly if/when the type/interface in question evolves without a revisit of the guard (e.g. adding a property).

It would be nice to be able to disallow all type guards, and/or to disallow ones asserting compatibility with a named type or interface (as opposed to an inline type).