LibCore+AK: Move MappedFile from AK to LibCore

MappedFile is strictly a userspace thing, so it doesn't belong in AK
(which is supposed to be user/kernel agnostic.)

Userland/Libraries/LibCore/MappedFile.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/Noncopyable.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+#include <AK/Result.h>
+
+namespace Core {
+
+class MappedFile : public RefCounted<MappedFile> {
+    AK_MAKE_NONCOPYABLE(MappedFile);
+    AK_MAKE_NONMOVABLE(MappedFile);
+
+public:
+    static ErrorOr<NonnullRefPtr<MappedFile>> map(String const& path);
+    static ErrorOr<NonnullRefPtr<MappedFile>> map_from_fd_and_close(int fd, String const& path);
+    ~MappedFile();
+
+    void* data() { return m_data; }
+    const void* data() const { return m_data; }
+
+    size_t size() const { return m_size; }
+
+    ReadonlyBytes bytes() const { return { m_data, m_size }; }
+
+private:
+    explicit MappedFile(void*, size_t);
+
+    void* m_data { nullptr };
+    size_t m_size { 0 };
+};
+
+}