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/KBufferBuilder.h

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/StringBuilder.h>
+#include <AK/StringView.h>
+#include <Kernel/Library/KBuffer.h>
+
+namespace Kernel {
+
+class KBufferBuilder {
+    AK_MAKE_NONCOPYABLE(KBufferBuilder);
+
+public:
+    using OutputType = KBuffer;
+
+    static ErrorOr<KBufferBuilder> try_create();
+
+    KBufferBuilder(KBufferBuilder&&) = default;
+    KBufferBuilder& operator=(KBufferBuilder&&) = default;
+    ~KBufferBuilder() = default;
+
+    ErrorOr<void> append(StringView);
+    ErrorOr<void> append(char);
+    ErrorOr<void> append(char const*, int);
+
+    ErrorOr<void> append_escaped_for_json(StringView);
+    ErrorOr<void> append_bytes(ReadonlyBytes);
+
+    template<typename... Parameters>
+    ErrorOr<void> appendff(CheckedFormatString<Parameters...>&& fmtstr, Parameters const&... parameters)
+    {
+        // FIXME: This really not ideal, but vformat expects StringBuilder.
+        StringBuilder builder;
+        AK::VariadicFormatParams<AK::AllowDebugOnlyFormatters::No, Parameters...> variadic_format_params { parameters... };
+        TRY(vformat(builder, fmtstr.view(), variadic_format_params));
+        return append_bytes(builder.string_view().bytes());
+    }
+
+    bool flush();
+    OwnPtr<KBuffer> build();
+
+    ReadonlyBytes bytes() const
+    {
+        if (!m_buffer)
+            return {};
+        return m_buffer->bytes();
+    }
+
+    size_t length() const
+    {
+        return m_size;
+    }
+
+private:
+    explicit KBufferBuilder(NonnullOwnPtr<KBuffer>);
+
+    bool check_expand(size_t);
+    u8* insertion_ptr()
+    {
+        if (!m_buffer)
+            return nullptr;
+        return m_buffer->data() + m_size;
+    }
+
+    OwnPtr<KBuffer> m_buffer;
+    size_t m_size { 0 };
+};
+
+}