Add primitive FIFO and hook it up to sys$pipe().

It's now possible to do this in bash:

cat kernel.map | fgrep List

This is very cool! :^)

Kernel/FIFO.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <AK/CircularQueue.h>
+#include <AK/Retainable.h>
+#include <AK/RetainPtr.h>
+#include <VirtualFileSystem/UnixTypes.h>
+
+class FIFO : public Retainable<FIFO> {
+public:
+    enum Direction {
+        Neither, Reader, Writer
+    };
+
+    static RetainPtr<FIFO> create();
+
+    void open(Direction);
+    void close(Direction);
+
+    Unix::ssize_t write(const byte*, Unix::size_t);
+    Unix::ssize_t read(byte*, Unix::size_t);
+
+    bool can_read() const;
+    bool can_write() const;
+
+private:
+    FIFO();
+
+    unsigned m_writers { 0 };
+    unsigned m_readers { 0 };
+    CircularQueue<byte, 16> m_queue;
+};