Everywhere: Move global Kernel pattern code to Kernel/Library directory

This has KString, KBuffer, DoubleBuffer, KBufferBuilder, IOWindow,
UserOrKernelBuffer and ScopedCritical classes being moved to the
Kernel/Library subdirectory.

Also, move the panic and assertions handling code to that directory.

Kernel/Library/DoubleBuffer.h

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <Kernel/Library/KBuffer.h>
+#include <Kernel/Library/UserOrKernelBuffer.h>
+#include <Kernel/Locking/Mutex.h>
+#include <Kernel/Tasks/Thread.h>
+
+namespace Kernel {
+
+class DoubleBuffer {
+public:
+    static ErrorOr<NonnullOwnPtr<DoubleBuffer>> try_create(StringView name, size_t capacity = 65536);
+    ErrorOr<size_t> write(UserOrKernelBuffer const&, size_t);
+    ErrorOr<size_t> write(u8 const* data, size_t size)
+    {
+        return write(UserOrKernelBuffer::for_kernel_buffer(const_cast<u8*>(data)), size);
+    }
+    ErrorOr<size_t> read(UserOrKernelBuffer&, size_t);
+    ErrorOr<size_t> read(u8* data, size_t size)
+    {
+        auto buffer = UserOrKernelBuffer::for_kernel_buffer(data);
+        return read(buffer, size);
+    }
+    ErrorOr<size_t> peek(UserOrKernelBuffer&, size_t);
+    ErrorOr<size_t> peek(u8* data, size_t size)
+    {
+        auto buffer = UserOrKernelBuffer::for_kernel_buffer(data);
+        return peek(buffer, size);
+    }
+
+    bool is_empty() const { return m_empty; }
+
+    size_t space_for_writing() const { return m_space_for_writing; }
+    size_t immediately_readable() const
+    {
+        return (m_read_buffer->size - m_read_buffer_index) + m_write_buffer->size;
+    }
+
+    void set_unblock_callback(Function<void()> callback)
+    {
+        VERIFY(!m_unblock_callback);
+        m_unblock_callback = move(callback);
+    }
+
+private:
+    explicit DoubleBuffer(size_t capacity, NonnullOwnPtr<KBuffer> storage);
+    void flip();
+    void compute_lockfree_metadata();
+
+    ErrorOr<size_t> read_impl(UserOrKernelBuffer&, size_t, MutexLocker&, bool advance_buffer_index);
+
+    struct InnerBuffer {
+        u8* data { nullptr };
+        size_t size { 0 };
+    };
+
+    InnerBuffer* m_write_buffer { nullptr };
+    InnerBuffer* m_read_buffer { nullptr };
+    InnerBuffer m_buffer1;
+    InnerBuffer m_buffer2;
+
+    NonnullOwnPtr<KBuffer> m_storage;
+    Function<void()> m_unblock_callback;
+    size_t m_capacity { 0 };
+    size_t m_read_buffer_index { 0 };
+    size_t m_space_for_writing { 0 };
+    bool m_empty { true };
+    mutable Mutex m_lock { "DoubleBuffer"sv };
+};
+
+}