Get rid of special include orders and forward declarations

[eggrobin]

Oddities such as the ones listed below are deeply confusing already, and are obviously module-hostile. There are probably better ways to deal with that.

Principia/quantities/generators.hpp

Lines 29 to 30 in b0bec96

	// Because of circular dependencies, this file doesn't include
	// generators_body.hpp. This will be done by quantities.hpp.

Principia/quantities/quantities.hpp

Lines 176 to 179 in b0bec96

	// Include before quantities_body.hpp all the bodies that want to see the
	// definition of class Quantity.
	#include "quantities/generators_body.hpp"
	#include "quantities/wide_body.hpp"

Cross-module forward declarations are impossible, so we should probably try to minimize forward declarations as well.