Avoid cyclic dependencies among source files

Reason

Cycles complicate comprehension and slow down compilation. They also complicate conversion to use language-supported modules (when they become available).

Note

Eliminate cycles; don't just break them with #include guards.

Example, bad

// file1.h:
#include "file2.h"

// file2.h:
#include "file3.h"

// file3.h:
#include "file1.h"

Enforcement

Flag all cycles.