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

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.

New dolphin-tool command: "header"
-b / --block_size
-c / --compression
-l / --compression_level

Informative RVZ/WIA header2 value "compression_level" is now a s32 instead of a u32, because negative compression is a thing.

Speaking of, it is now possible to use negative compression levels in dolphin-tool's convert command (not the GUI, though).

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.