refactor: IWYU applied to the source files

`alternative_namespaces` examples left out as I consider them a subject for removal in the nearest future.

example/glide_computer.cpp

@@ -21,9 +21,13 @@
 // SOFTWARE.
 
 #include "glide_computer.h"
+#include <numeric>
+#include <string_view>
 
 namespace glide_computer {
 
+using namespace units::isq;
+
 task::legs task::make_legs(const waypoints& wpts)
 {
   task::legs res;