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/KString.cpp

@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Format.h>
+#include <AK/StringBuilder.h>
+#include <Kernel/Library/KString.h>
+
+extern bool g_in_early_boot;
+
+namespace Kernel {
+
+ErrorOr<NonnullOwnPtr<KString>> KString::try_create(StringView string)
+{
+    char* characters = nullptr;
+    size_t length = string.length();
+    auto new_string = TRY(KString::try_create_uninitialized(length, characters));
+    if (!string.is_empty())
+        __builtin_memcpy(characters, string.characters_without_null_termination(), length);
+    characters[length] = '\0';
+    return new_string;
+}
+
+ErrorOr<NonnullOwnPtr<KString>> KString::vformatted(StringView fmtstr, AK::TypeErasedFormatParams& params)
+{
+    StringBuilder builder;
+    TRY(AK::vformat(builder, fmtstr, params));
+    return try_create(builder.string_view());
+}
+
+NonnullOwnPtr<KString> KString::must_create(StringView string)
+{
+    // We can only enforce success during early boot.
+    VERIFY(g_in_early_boot);
+    return KString::try_create(string).release_value();
+}
+
+ErrorOr<NonnullOwnPtr<KString>> KString::try_create_uninitialized(size_t length, char*& characters)
+{
+    size_t allocation_size = sizeof(KString) + (sizeof(char) * length) + sizeof(char);
+    auto* slot = kmalloc(allocation_size);
+    if (!slot)
+        return ENOMEM;
+    auto new_string = TRY(adopt_nonnull_own_or_enomem(new (slot) KString(length)));
+    characters = new_string->m_characters;
+    return new_string;
+}
+
+NonnullOwnPtr<KString> KString::must_create_uninitialized(size_t length, char*& characters)
+{
+    // We can only enforce success during early boot.
+    VERIFY(g_in_early_boot);
+    return KString::try_create_uninitialized(length, characters).release_value();
+}
+
+ErrorOr<NonnullOwnPtr<KString>> KString::try_clone() const
+{
+    return try_create(view());
+}
+
+void KString::operator delete(void* string)
+{
+    if (!string)
+        return;
+    size_t allocation_size = sizeof(KString) + (sizeof(char) * static_cast<KString*>(string)->m_length) + sizeof(char);
+    kfree_sized(string, allocation_size);
+}
+
+}