Kernel: Add KBufferBuilder, similar to StringBuilder but for KBuffer

This class works by eagerly allocating 1MB of virtual memory but only
adding physical pages on demand. In other words, when you append to it,
its memory usage will increase by 1 page whenever you append across a
page boundary (4KB.)

Kernel/KBufferBuilder.cpp

@@ -0,0 +1,66 @@
+#include <AK/PrintfImplementation.h>
+#include <AK/StdLibExtras.h>
+#include <KBufferBuilder.h>
+#include <stdarg.h>
+
+inline bool KBufferBuilder::can_append(size_t size) const
+{
+    bool has_space = ((m_size + size) < m_buffer.size());
+    ASSERT(has_space);
+    return has_space;
+}
+
+KBuffer KBufferBuilder::build()
+{
+    m_buffer.set_size(m_size);
+    return m_buffer;
+}
+
+KBufferBuilder::KBufferBuilder()
+    : m_buffer(KBuffer::create_with_size(1048576))
+{
+}
+
+void KBufferBuilder::append(const StringView& str)
+{
+    if (str.is_empty())
+        return;
+    if (!can_append(str.length()))
+        return;
+    memcpy(insertion_ptr(), str.characters_without_null_termination(), str.length());
+    m_size += str.length();
+}
+
+void KBufferBuilder::append(const char* characters, int length)
+{
+    if (!length)
+        return;
+    if (!can_append(length))
+        return;
+    memcpy(insertion_ptr() + m_size, characters, length);
+    m_size += length;
+}
+
+void KBufferBuilder::append(char ch)
+{
+    if (!can_append(1))
+        return;
+    insertion_ptr()[0] = ch;
+    m_size += 1;
+}
+
+void KBufferBuilder::appendvf(const char* fmt, va_list ap)
+{
+    printf_internal([this](char*&, char ch) {
+        append(ch);
+    },
+        nullptr, fmt, ap);
+}
+
+void KBufferBuilder::appendf(const char* fmt, ...)
+{
+    va_list ap;
+    va_start(ap, fmt);
+    appendvf(fmt, ap);
+    va_end(ap);
+}