Add macro methods for Metaclass nodes

[HertzDevil]

c.f. #3274

Every Metaclass node can be reconstituted via:

macro foo(x)
  ({{ x.instance }}).class
end

Passing a T.class to a macro directly would produce a Call node instead of a Metaclass node, because macro arguments are interpreted as non-type expressions. Nonetheless, Metaclass nodes may appear in def restrictions or TypeDeclarations; since they are valid type names, they also have the #resolve and #resolve? methods.

[straight-shoota]

Instantiating this node is cheap. I'd prefer to do that in every example to keep them self-contained.