Changes to the hasSubsequence answer

- Ensure that `startsWith` is tail-recursive
- Update `hasSubsequence` signature to match the one shown in the book
- Use _ placeholder in `hasSubsequence`

In addition, my text editor removed the trailing whitespaces of the 2 files. I think it's an improvement overall, but I split the commit in 2 in case you want me to get rid of it.