Allow Moo attributes to be "thunked"

 MooX::Thunking is a Moo extension allowing another value for the "is" parameter
 to "has" in Moo: "thunked". If used, this will allow you to transparently
 provide either a real value for the attribute, or a "CodeLike" in
 Types::TypeTiny that when called will return such a real value.