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/Common/SysConf.cpp

@@ -56,8 +56,7 @@ void SysConf::Load()
 {
   Clear();
 
-  if (!File::Exists(m_file_name) || File::GetSize(m_file_name) != SYSCONF_SIZE ||
-      !LoadFromFile(m_file_name))
+  if (File::GetSize(m_file_name) != SYSCONF_SIZE || !LoadFromFile(m_file_name))
   {
     WARN_LOG(CORE, "No valid SYSCONF detected. Creating a new one.");
     InsertDefaultEntries();