Add a public API for checking values against Value annotations

[chrispyles]

As titled. For example, to check whether a value annotation is satisfied by some object:

v = pybryt.Value(obj)
v.check_against(other_obj)

Allows instructors to test the robustness of their annotations and engage in unit testing assignments. Also useful for demonstration purposes.

Any method implementing this feature should just wrap the object in the tuple abstraction for a memory footprint observed value and use existing methods to validate the pseudo-footprint.

[chrispyles]

A similar method should also be added to the Attribute annotation.