Core & UnitTests: Make overriding explicit and remove redundant virtual specifiers on overriding destructors

DiscIO: Pass parameters by constant reference

A deep-copy method CopyReader has been added to BlobReader (virtual) and all of its subclasses (override). This should create a second BlobReader to open the same set of data but with an independent read pointer so that it doesn't interfere with any reads done on the original Reader.

As part of this, IOFile has added code to create a deep copy IOFile pointer onto the same file, with code based on the platform in question to find the file ID from the file pointer and open a new one. There has also been a small piece added to FileInfo to enable a deep copy, but its only subclass at this time already had a copy constructor so this was relatively minor.

Previously, we had WBFS and CISO which both returned an upper bound
of the size, and other formats which returned an accurate size. But
now we also have NFS, which returns a lower bound of the size. To
allow VolumeVerifier to make better informed decisions for NFS, let's
use an enum instead of a bool for the type of data size a blob has.

For a few years now, I've been thinking it would be nice to make Dolphin
support reading Wii games in the format they come in when you download
them from the Wii U eShop. The Wii U eShop has some good deals on Wii
games (Metroid Prime Trilogy especially is rather expensive if you try
to buy it physically!), and it's the only place right now where you can
buy Wii games digitally.

Of course, Nintendo being Nintendo, next year they're going to shut down
this only place where you can buy Wii games digitally. I kind of wish I
had implemented this feature earlier so that people would've had ample
time to buy the games they want, but... better late than never, right?

I used MIT-licensed code from the NOD library as a reference when
implementing this. None of the code has been directly copied, but
you may notice that the names of the struct members are very similar.
https://gitlab.axiodl.com/AxioDL/nod/blob/c1635245b881ed0004ff5e616896579ce1b19164/lib/DiscIONFS.cpp