loaders: Add SubIO().

SubIO is an interface to create an I/O (given a parent I/O) that is
restricted to a continuous I/O subinterval.

TODO: Docs + tests.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: PSD: More work on the PSD loaders.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>