FileUtil: Add a class for Exists/IsDirectory/GetSize

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.

Source/Core/DiscIO/WiiWad.cpp

@@ -46,7 +46,7 @@ bool IsWiiWAD(BlobReader& reader)
 
 WiiWAD::WiiWAD(const std::string& name) : m_reader(CreateBlobReader(name))
 {
-  if (m_reader == nullptr || File::IsDirectory(name))
+  if (m_reader == nullptr)
   {
     m_valid = false;
     return;