Introduce Textpattern_Iterator_FileIterator.

This chunk reading iterator will act as the base for
newer, memory efficient Textpack parser and
language interface among other things.

The class name and what current() returns may later on
still change. If we ever get string object, current will return
that. Name may change if the location changes, e.g. if
we get stack of filesystem functions to core.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5524 2fea6a4d-a838-0410-917b-93a53c48e9d2