Deprecate the `file_size()` function

[Moohan]

This resolves #11

As far as we can tell file_size() is now redundant.

This PR deprecates the function and removes it from the package documentation, if no one objects we can remove it in a future version. For now file_size() will still work but will generate a warning every time, ?file_size will also bring up the function help but the help won't appear in the function reference.