Make comments more consistent

- Changes most `/* comments */` to `// comments`
- Changes `/**` block comments consistently to `/*`
- Adds consistent license comments to all files

Also renames `T_POP_SET` to `T_Z80_SET`

include/link/assign.h

@@ -6,7 +6,7 @@
  * SPDX-License-Identifier: MIT
  */
 
-/* Assigning all sections a place */
+// Assigning all sections a place
 #ifndef RGBDS_LINK_ASSIGN_H
 #define RGBDS_LINK_ASSIGN_H
 
@@ -14,14 +14,10 @@
 
 extern uint64_t nbSectionsToAssign;
 
-/**
- * Assigns all sections a slice of the address space
- */
+// Assigns all sections a slice of the address space
 void assign_AssignSections(void);
 
-/**
- * `free`s all assignment memory that was allocated.
- */
+// `free`s all assignment memory that was allocated
 void assign_Cleanup(void);
 
-#endif /* RGBDS_LINK_ASSIGN_H */
+#endif // RGBDS_LINK_ASSIGN_H