Expand the structs and traits in src/syscalls/fs.rs

[simonschoening]

We do want to support directories.

Existing functions in fuse.rs have to be adapted (e. g. lookup).

[simonschoening]

I think we might want to adapt our implementation to the Rust std library:
https://github.com/rust-lang/rust/blob/master/library/std/src/fs.rs