Implement /proc/PID/vm.

Refactored SyntheticFileSystem to maintain an arbitrary directory structure.
ProcFileSystem creates a directory entry in /proc for each new process.

Kernel/ProcFileSystem.h

@@ -3,15 +3,24 @@
 #include <AK/Types.h>
 #include <VirtualFileSystem/SyntheticFileSystem.h>
 
+class Task;
+
 class ProcFileSystem final : public SyntheticFileSystem {
 public:
+    static ProcFileSystem& the();
+
     virtual ~ProcFileSystem() override;
     static RetainPtr<ProcFileSystem> create();
 
     virtual bool initialize() override;
     virtual const char* className() const override;
 
+    void addProcess(Task&);
+    void removeProcess(Task&);
+
 private:
     ProcFileSystem();
+
+    HashMap<pid_t, InodeIndex> m_pid2inode;
 };