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`

src/opmath.c

@@ -6,9 +6,7 @@
  * SPDX-License-Identifier: MIT
  */
 
-/*
- * Mathematical operators that don't reuse C's behavior
- */
+// Mathematical operators that don't reuse C's behavior
 
 #include <stdint.h>