FileSystem: Add a Custody class that represents a parent/child guardianship.

A custody is kind of a directory entry abstraction that represents a single
entry in a parent directory that tells us the name of a child inode.

The idea here is for path resolution to produce a chain of custody objects.

Kernel/FileSystem/Custody.cpp

@@ -0,0 +1,27 @@
+#include <AK/HashTable.h>
+#include <Kernel/FileSystem/Custody.h>
+#include <Kernel/FileSystem/Inode.h>
+#include <Kernel/Lock.h>
+
+static Lockable<HashTable<Custody*>>& all_custodies()
+{
+    static Lockable<HashTable<Custody*>>* table;
+    if (!table)
+        table = new Lockable<HashTable<Custody*>>;
+    return *table;
+}
+
+Custody::Custody(Custody* parent, const String& name, Inode& inode)
+    : m_parent(parent)
+    , m_name(name)
+    , m_inode(inode)
+{
+    LOCKER(all_custodies().lock());
+    all_custodies().resource().set(this);
+}
+
+Custody::~Custody()
+{
+    LOCKER(all_custodies().lock());
+    all_custodies().resource().remove(this);
+}