Misc tasks

- [ ] 1. gcc -O3, clang and tcc are not tested
- [ ] 2. you could also use address sanitizer and ub sanitizer (although I would expect they dont find anything)
- [ ] 3. add example usage for multiple types in the README
- [ ] 4. Did you consider using `uint8_t` instead of `int` ?
- [x] 5. create a license or SPDX header.