provide a way to tell the typechecker that a string classname definitely implements an interface

[fredemmott]

eg:

$classes = $all_classes->filter($class => is_subclass_of($class, SomeInterface));
foreach ($classes as $class) {
  $class::someStaticFunc();
}

[fredemmott]

assert($class instanceof SomeInterface) makes the typechecker happy, but is invalid at runtime.

[lexidor]

This would be nice to have in the runtime, to be able to refine is an is like way, instead of an as like way.
\Facebook\TypeSpec\classname(IFace::class)->assertType($some_classname) is what you can do today.
Maybe $x is IFace::class could be taught to perform this trick.
This could not have been possible with the old instanceof operator, since that would allow a stringname of a class on the RHS, which would have made checks for classname<T> and T ambiguous.

The repro would never work though, since the typechecker does not understand that all elements of $classes meet the predicate, since it does not understand \ConstCollection::filter().