Handle `Struct.new`, `Data.define`, `Class.new` and `Module.new`

Putting all of these in the same issue as I suspect there'll be some shared logic. Using these methods return classes or modules and if we set those to a constant, they'll become a namespace where modules can be included, definitions can be created and so on.

In all cases where we write a constant, we need to check if the value is a new call node with a receiver of Struct, Class, Module (potentially OpenStruct too) and then create a class or module instead of a constant.

Important caveat: while invoking these methods does produce new classes and modules, they do not produce a new lexical scope like the class or module keyword. For example:

class Foo
  Bar = Struct.new do
    CONST = 123
  end
end

The CONST definition here is Foo::CONST because Bar does not advance the lexical scope. In practical terms, it means we need to insert a ClassDefinition or ModuleDefinition in the graph, but we should not push anything to the definition stack.