Fixing issue #3 and #4 on GitHub.

Issue #3: calling FILE-POSITION instead of STREAM-FILE-POSITION, per
trivial-gray-streams maintainer request.

Issue #4: flexi-streams was not updating its notion of POSITION.
Wanted to be defer handling of :start/:end to underlying stream,
so the setter method of STREAM-FILE-POSITION calls FILE-POSITION
(aka Issue #3), then if that returns a non-nil value, calls
FILE-POSITION to find what POSITION should be set to.