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/script.h

@@ -6,7 +6,7 @@
  * SPDX-License-Identifier: MIT
  */
 
-/* Parsing a linker script */
+// Parsing a linker script
 #ifndef RGBDS_LINK_SCRIPT_H
 #define RGBDS_LINK_SCRIPT_H
 
@@ -24,15 +24,15 @@ struct SectionPlacement {
 
 extern uint64_t script_lineNo;
 
-/**
+/*
  * Parses the linker script to return the next section constraint
  * @return A pointer to a struct, or NULL on EOF. The pointer shouldn't be freed
  */
 struct SectionPlacement *script_NextSection(void);
 
-/**
+/*
  * `free`s all assignment memory that was allocated.
  */
 void script_Cleanup(void);
 
-#endif /* RGBDS_LINK_SCRIPT_H */
+#endif // RGBDS_LINK_SCRIPT_H