Some code was calling more than one of these functions in a row
(in particular, FileUtil.cpp itself did it a lot...), which is
a waste since it's possible to call stat a single time and then
read all three values from the stat struct. This commit adds a
File::FileInfo class that calls stat once on construction and
then lets Exists/IsDirectory/GetSize be executed very quickly.

The performance improvement mostly matters for functions that
can be handling a lot of files, such as File::ScanDirectoryTree.

I've also done some cleanup in code that uses these functions.
For instance, some code had checks like !Exists() || !IsDirectory(),
which is functionally equivalent to !IsDirectory(), and some
code was using File::GetSize even though there was an IOFile
object that the code could call GetSize on.

That is the official term used by Nintendo.

It's not really a BIOS.

GCMemcard.h has quite a bit of different classes implemented within it
that could likely be split up into other files to make it a little
easier to read. However, they should be moved into their own folder
first so that they don't clutter up the base HW directory.

Keeps related source files together and cleans up the root directory of HW
a little bit.