Previously, the type parameter of the kind of node that was checked for
needed to be passed.
This node kind had to be matched by the given runtime `test`.
At that point, it was known that node would be of that kind.

Now, the given runtime test is used in the type system to narrow the
given `node` value down.

This means that the types can become much simpler.
Instead of `AssertAnything` and `AssertPredicate`, use the new `Check`.
Instead of `TestFunctionAnything` and `TestFunctionPredicate`, use
`TestFunction`.
Instead of `PredicateTest`, use `Test`.