Kernel: Refactor MemoryManager to use a Bitmap rather than a Vector

This significantly reduces the pressure on the kernel heap when
allocating a lot of pages.

Previously at about 250MB allocated, the free page list would outgrow
the kernel's heap. Given that there is no longer a page list, this does
not happen.

The next barrier will be the kernel memory used by the page records for
in-use memory. This kicks in at about 1GB.

Kernel/VM/PhysicalRegion.cpp

@@ -0,0 +1,77 @@
+#include <AK/Bitmap.h>
+#include <AK/Retained.h>
+#include <AK/RetainPtr.h>
+#include <Kernel/Assertions.h>
+#include <Kernel/PhysicalAddress.h>
+#include <Kernel/VM/PhysicalPage.h>
+#include <Kernel/VM/PhysicalRegion.h>
+
+Retained<PhysicalRegion> PhysicalRegion::create(PhysicalAddress lower, PhysicalAddress upper)
+{
+    return adopt(*new PhysicalRegion(lower, upper));
+}
+
+PhysicalRegion::PhysicalRegion(PhysicalAddress lower, PhysicalAddress upper)
+    : m_lower(lower)
+    , m_upper(upper)
+    , m_bitmap(Bitmap::create())
+{
+}
+
+void PhysicalRegion::expand(PhysicalAddress lower, PhysicalAddress upper)
+{
+    ASSERT(!m_pages);
+
+    m_lower = lower;
+    m_upper = upper;
+}
+
+unsigned PhysicalRegion::finalize_capacity()
+{
+    ASSERT(!m_pages);
+
+    m_pages = (m_upper.get() - m_lower.get()) / PAGE_SIZE;
+    m_bitmap.grow(m_pages, false);
+
+    return size();
+}
+
+RetainPtr<PhysicalPage> PhysicalRegion::take_free_page(bool supervisor)
+{
+    ASSERT(m_pages);
+
+    if (m_used == m_pages)
+        return nullptr;
+
+    for (unsigned page = m_last; page < m_pages; page++) {
+        if (!m_bitmap.get(page)) {
+            m_bitmap.set(page, true);
+            m_used++;
+            m_last = page + 1;
+            return PhysicalPage::create(m_lower.offset(page * PAGE_SIZE), supervisor);
+        }
+    }
+
+    ASSERT_NOT_REACHED();
+
+    return nullptr;
+}
+
+void PhysicalRegion::return_page_at(PhysicalAddress addr)
+{
+    ASSERT(m_pages);
+
+    if (m_used == 0) {
+        ASSERT_NOT_REACHED();
+    }
+
+    int local_offset = addr.get() - m_lower.get();
+    ASSERT(local_offset >= 0);
+    ASSERT(local_offset < m_pages * PAGE_SIZE);
+
+    auto page = local_offset / PAGE_SIZE;
+    if (page < m_last) m_last = page;
+
+    m_bitmap.set(page, false);
+    m_used--;
+}