add (setf file-position) function

On parsing large binary file it's very inconvenient to load it completely in the memory for access complex structures.

For example:
In "Microsoft Compound File" internal FAT can be at any part of the file and sectors can be in any order like in FAT16(32) file system.
And if I want to load only the structure tree without data stream Then I want to get data from stream I must open and load it again.

This extension can be realized in any modern operating system and seems to be equivalent to (setf stream-element-type) - that is not realized in SBCL for example :-).
Yes the code using this will be unportable but it's Very Very convenient and intuitive.