Add more file functions to stdlib

So far, the only function for file paths is `std.thisFile`, but its utility is questionable.

We need functions giving us the full pathname of the current file, just the filename, and just the directory. It would be even better if we can get a function that extracts the filename, and the directory of a given file path, too.

I've been trying to use `importstr` and include configuration files, but I cannot make it work when the containing file is symlinked and it uses relative to its configuration files. Of course, the same applies to `import`, too.