allign = and unless & fix wrong emptyish test
first attempt at parsing & writing storage.config

hitting a snag here because apparently I cannot inherit from a
ParsedFile and make it actually work. I'm stumped.

aside from that: I'm a bit unhappy about my choice of putting validation
in the provider's :post_parse block. It belongs in the type. I think I
should add a `provider` attribute (newattr?), that binds `size`:

    iff `provider` is `directory`, then `size` must be set.
    iff `provider` is `raw`, then `size` must not be set.