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.