Common new-type wrapper for more precise arguments

[aomader]

Writing a CLI it is quite common to work with paths. However, there is currently no easy and default way to specify certain constraints without implementing something individual. Therefore, I propose to implement common new-type wrappers to support the most used constraints on CLI arguments.

From the top of my head I think about something like these types, with appropriate Decodable implementations:

• struct ExistingFile(PathBuf);
• struct WritableFile(PathBuf);
• struct WritableDirectory(PathBuf);

This seems as it would solve most of current issues like #152, #135.