public interface Assertion
Marker trait for ScalaTest-compatible assertion types.
This will eventually be released as a module, so that third-party libraries
can depend just on a module that has just this one trait in it. Since it
compiles down to a simple Java interface, there should be no binary compatibility
worries. It allows third party libraries to define assertions that can appear
last in a ScalaTest test body that requires type `compatible.Assertion`.