Various small cleanups in FileStorage.

Share backpointer implementation between _loadBack() and
_loadBackPOS() to avoid duplicate code.

Simplify interface between FileIterator and RecordIterator.  Don't
pass unused arguments and don't pass nested tuples.

Use global variables to hold trans and data struct formats.

Reformat several docstrings.

Add whitespace to RecordIterator so that I can read it.