Use header files for all declarations used in multiple source files

Reason

Maintainability. Readability.

Example, bad

// bar.cpp:
void bar() { cout << "bar\n"; }

// foo.cpp:
extern void bar();
void foo() { bar(); }

A maintainer of bar cannot find all declarations of bar if its type needs changing. The user of bar cannot know if the interface used is complete and correct. At best, error messages come (late) from the linker.

Enforcement

• Flag declarations of entities in other source files not placed in a .h.