cfunction (file/seek f &opt whence n) Jump to a relative location in the file `f`. `whence` must be one of: * :cur - jump relative to the current file location * :set - jump relative to the beginning of the file * :end - jump relative to the end of the file By default, `whence` is :cur. Optionally a value `n` may be passed for the relative number of bytes to seek in the file. `n` may be a real number to handle large files of more than 4GB. Returns the file handle.