Fix invalid `fs/touch` return value

`fs/touch` should have been returning the file object but instead was
returning the `fs/file` function.
Documentation clean up.
Add `fs/hidden?` for testing if files are hidden
Closes #48: Add `fs/link?` for testing if a file is a symbolic link

Also added functions `fs/link`and `fs/sym-link` for creating hard and
soft links respectively.