Add a very naive block cache to the DiskBackedFileSystem.

This would be a lot better as an LRU. Right now it's a 32-slot
hash table with random eviction.

VirtualFileSystem/DiskBackedFileSystem.h

@@ -2,6 +2,7 @@
 
 #include "FileSystem.h"
 #include <AK/ByteBuffer.h>
+#include <AK/HashMap.h>
 
 class DiskBackedFileSystem : public FileSystem {
 public:
@@ -27,4 +28,5 @@ protected:
 private:
     unsigned m_blockSize { 0 };
     RetainPtr<DiskDevice> m_device;
+    mutable HashMap<unsigned, ByteBuffer> m_blockCache;
 };