It's useful to be able to seek to a specific position in a file. Allow
AutoFile to seek by using fseek.
It's also useful to be able to get the current position in a file.
Allow AutoFile to tell by using ftell.
New code can call the method without having first to retrieve the raw
FILE* pointer via Get().
Also, move implementation to the cpp file. Can be reviewed with:
--color-moved=dimmed-zebra --color-moved-ws=ignore-all-space
