Use a File class to wrap file compilation logic

Steps to take:

- [ ] Add a simple File class only containing four basic attributes. Something like:
 ~~~perl6
 class File {

    has Status  $.status;
    has Str     $.path;
    has Instant $.added;
    has Str     $.cache-key;

}
 ~~~
- [ ] Integrate it in the system.
- [ ] Add methods to compile the file, etc.