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/Core/Boot/Boot.cpp

@@ -466,8 +466,8 @@ bool CBoot::BootUp(std::unique_ptr<BootParameters> boot)
 
 BootExecutableReader::BootExecutableReader(const std::string& file_name)
 {
-  m_bytes.resize(File::GetSize(file_name));
   File::IOFile file{file_name, "rb"};
+  m_bytes.resize(file.GetSize());
   file.ReadBytes(m_bytes.data(), m_bytes.size());
 }