[kernel] Implement VMA page tracking

The previous method of VMA page tracking relied on the VMA always being
mapped at least into one space and just kept track of pages in the
spaces' page tables. This had a number of drawbacks, and the mapper
system was too complex without much benefit.

Now make VMAs themselves keep track of spaces that they're a part of,
and make them responsible for knowing what page goes where. This
simplifies most types of VMA greatly. The new vm_area_open (nee
vm_area_shared, but there is now no reason for most VMAs to be
explicitly shareable) adds a 64-ary radix tree for tracking allocated
pages.

The page_tree cannot yet handle taking pages away, but this isn't
something jsix can do yet anyway.

src/kernel/page_tree.h

@@ -0,0 +1,39 @@
+#pragma once
+/// \file page_tree.h
+/// Definition of mapped page tracking structure and related definitions
+
+#include <stdint.h>
+
+/// A radix tree node that tracks mapped pages
+class page_tree
+{
+public:
+	/// Get the physical address of the page at the given offset.
+	/// \arg root    The root node of the tree
+	/// \arg offset  Offset into the VMA, in bytes
+	/// \arg page    [out] Receives the page physical address, if found
+	/// \returns     True if a page was found
+	static bool find(const page_tree *root, uint64_t offset, uintptr_t &page);
+
+	/// Get the physical address of the page at the given offset. If one does
+	/// not exist yet, allocate a page, insert it, and return that.
+	/// \arg root    [inout] The root node of the tree. This pointer may be updated.
+	/// \arg offset  Offset into the VMA, in bytes
+	/// \arg page    [out] Receives the page physical address, if found
+	/// \returns     True if a page was found
+	static bool find_or_add(page_tree * &root, uint64_t offset, uintptr_t &page);
+
+private:
+	page_tree(uint64_t base, uint8_t level);
+
+	/// Stores the page offset of the start of this node's pages in bits 0:41
+	/// and the depth of tree this node represents in bits 42:44 (0-7)
+	uint64_t m_base;
+
+	/// For a level 0 node, the entries area all physical page addresses.
+	/// Other nodes contain pointers to child tree nodes.
+	union {
+		uintptr_t entry;
+		page_tree *child;
+	} m_entries[64];
+};