This gets rid of some assumptions that non-DiscIO code was making about
volume types. It's better to encapsulate as many of the volume type
differences as possible in DiscIO.
Made possible by PR #2353.
By removing mutable state in VolumeWiiCrypted, this change makes
partition-related code simpler. It also gets rid of other ugly things,
like ISOProperties's "over 9000" loop that creates a list of
partitions by trying possible combinations, and DiscScrubber's
volume swapping that recreates the entire volume when it needs to
change partition.
JosJuice