Refer to T.class as "metaclass" in error messages, not "class"

[HertzDevil]

Metaclasses are special in that they are classes but inherit from Value other than Reference. This produces some nonsensical error messages:

class Foo
end

alias A = Foo.class

class B < A # Error: Foo.class is not a class, it's a class
end

This PR makes it so that they are reported as "metaclass" instead. Doing so also fixes an internal compiler error:

class Foo
end

instance_sizeof(Foo.class) # BUG: called llvm_struct_type for Foo.class

[sdogruyol]

Thank you @HertzDevil 🙏