Kernel/LibC: Implement basic epoll

This implementation is on top of inodes instead of fds as linux does it.
If I start finding ports/software that relies on epoll allowing
duplicate inodes, I will do what linux does.

I'm probably missing multiple epoll_notify's which may cause hangs but
the system seems to work fine :dd:

kernel/include/kernel/Epoll.h

@@ -0,0 +1,68 @@
+#pragma once
+
+#include <BAN/CircularQueue.h>
+#include <BAN/HashMap.h>
+#include <BAN/HashSet.h>
+#include <kernel/FS/Inode.h>
+
+#include <sys/epoll.h>
+
+namespace Kernel
+{
+
+	class Epoll final : public Inode
+	{
+	public:
+		static BAN::ErrorOr<BAN::RefPtr<Epoll>> create();
+		~Epoll();
+
+		BAN::ErrorOr<void> ctl(int op, BAN::RefPtr<Inode> inode, epoll_event event);
+		BAN::ErrorOr<size_t> wait(BAN::Span<epoll_event> events, uint64_t waketime_ns);
+
+		void notify(BAN::RefPtr<Inode> inode, uint32_t event);
+
+	private:
+		Epoll() = default;
+
+	public:
+		ino_t ino() const override         { return 0; }
+		Mode mode() const override         { return { Mode::IRUSR | Mode::IWUSR }; }
+		nlink_t nlink() const override     { return 0; }
+		uid_t uid() const override         { return 0; }
+		gid_t gid() const override         { return 0; }
+		off_t size() const override        { return 0; }
+		timespec atime() const override    { return {}; }
+		timespec mtime() const override    { return {}; }
+		timespec ctime() const override    { return {}; }
+		blksize_t blksize() const override { return PAGE_SIZE; }
+		blkcnt_t blocks() const override   { return 0; }
+		dev_t dev() const override         { return 0; }
+		dev_t rdev() const override        { return 0; }
+
+		bool is_epoll() const override { return true; }
+
+		const FileSystem* filesystem() const override { return nullptr; }
+
+		bool can_read_impl() const override { return false; }
+		bool can_write_impl() const override { return false; }
+		bool has_error_impl() const override { return false; }
+		bool has_hangup_impl() const override { return false; }
+
+		BAN::ErrorOr<void> fsync_impl() override { return {}; }
+
+	private:
+		struct InodeRefPtrHash
+		{
+			BAN::hash_t operator()(const BAN::RefPtr<Inode>& inode)
+			{
+				return BAN::hash<const Inode*>()(inode.ptr());
+			}
+		};
+
+	private:
+		ThreadBlocker m_thread_blocker;
+		BAN::HashMap<BAN::RefPtr<Inode>, uint32_t,    InodeRefPtrHash> m_ready_events;
+		BAN::HashMap<BAN::RefPtr<Inode>, epoll_event, InodeRefPtrHash> m_listening_events;
+	};
+
+}