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/KBuffer.h

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+// KBuffer: Memory buffer backed by a kernel region.
+//
+// The memory is allocated via the global kernel-only page allocator, rather than via
+// kmalloc() which is what ByteBuffer/Vector/etc will use.
+//
+// This makes KBuffer a little heavier to allocate, but much better for large and/or
+// long-lived allocations, since they don't put all that weight and pressure on the
+// severely limited kmalloc heap.
+
+#include <AK/Assertions.h>
+#include <AK/StringView.h>
+#include <Kernel/Library/StdLib.h> // For memcpy. FIXME: Make memcpy less expensive to access a declaration of in the Kernel.
+#include <Kernel/Memory/MemoryManager.h>
+
+namespace Kernel {
+
+class [[nodiscard]] KBuffer {
+public:
+    static ErrorOr<NonnullOwnPtr<KBuffer>> try_create_with_size(StringView name, size_t size, Memory::Region::Access access = Memory::Region::Access::ReadWrite, AllocationStrategy strategy = AllocationStrategy::Reserve)
+    {
+        auto rounded_size = TRY(Memory::page_round_up(size));
+        auto region = TRY(MM.allocate_kernel_region(rounded_size, name, access, strategy));
+        return TRY(adopt_nonnull_own_or_enomem(new (nothrow) KBuffer { size, move(region) }));
+    }
+
+    static ErrorOr<NonnullOwnPtr<KBuffer>> try_create_with_bytes(StringView name, ReadonlyBytes bytes, Memory::Region::Access access = Memory::Region::Access::ReadWrite, AllocationStrategy strategy = AllocationStrategy::Reserve)
+    {
+        auto buffer = TRY(try_create_with_size(name, bytes.size(), access, strategy));
+        memcpy(buffer->data(), bytes.data(), bytes.size());
+        return buffer;
+    }
+
+    [[nodiscard]] u8* data() { return m_region->vaddr().as_ptr(); }
+    [[nodiscard]] u8 const* data() const { return m_region->vaddr().as_ptr(); }
+    [[nodiscard]] size_t size() const { return m_size; }
+    [[nodiscard]] size_t capacity() const { return m_region->size(); }
+
+    [[nodiscard]] ReadonlyBytes bytes() const { return { data(), size() }; }
+    [[nodiscard]] Bytes bytes() { return { data(), size() }; }
+
+    void set_size(size_t size)
+    {
+        VERIFY(size <= capacity());
+        m_size = size;
+    }
+
+private:
+    explicit KBuffer(size_t size, NonnullOwnPtr<Memory::Region> region)
+        : m_size(size)
+        , m_region(move(region))
+    {
+    }
+
+    size_t m_size { 0 };
+    NonnullOwnPtr<Memory::Region> m_region;
+};
+
+}