[boot] Improve bootloader allocation accounting

The bootloader relied on the kernel to know which parts of memory to not
allocate over. For the future shift of having the init process load
other processes instead of the kernel, the bootloader needs a mechanism
to just hand the kernel a list of allocations. This is now done through
the new bootloader allocator, which all allocation goes through. Pool
memory will not be tracked, and so can be overwritten - this means the
args structure and its other structures like programs need to be handled
right away, or copied by the kernel.

- Add bootloader allocator
- Implement a new linked-list based set of pages that act as allocation
  registers
- Allow for operator new in the bootloader, which goes through the
  global allocator for pool memory
- Split memory map and frame accouting code in the bootloader into
  separate memory_map.* files
- Remove many includes that could be replaced by forward declaration in
  the bootloader
- Add a new global template type, `counted`, which replaces the
  bootloader's `buffer` type, and updated kernel args structure to use it.
- Move bootloader's pointer_manipulation.h to the global include dir
- Make offset_iterator try to return references instead of pointers to
  make it more consistent with static array iteration
- Implement a stub atexit() in the bootloader to satisfy clang

src/include/pointer_manipulation.h

@@ -0,0 +1,41 @@
+/// \file pointer_manipulation.h
+/// Helper functions and types for doing type-safe byte-wise pointer math.
+#pragma once
+
+/// Return a pointer offset from `input` by `offset` bytes.
+/// \tparam T  Cast the return value to a pointer to `T`
+/// \tparam S  The type pointed to by the `input` pointer
+template <typename T, typename S>
+inline T* offset_ptr(S* input, ptrdiff_t offset) {
+	return reinterpret_cast<T*>(reinterpret_cast<uintptr_t>(input) + offset);
+}
+
+/// Iterator for an array of `T` whose size is known at runtime
+/// \tparam T  Type of the objects in the array, whose size might not be
+///            what is returned by sizeof(T).
+template <typename T>
+class offset_iterator
+{
+public:
+	/// Constructor.
+	/// \arg t    Pointer to the first item in the array
+	/// \arg off  Offset applied to reach successive items. Default is 0,
+	///           which creates an effectively constant iterator.
+	offset_iterator(T* t, size_t off=0) : m_t(t), m_off(off) {}
+
+	T* operator++() { m_t = offset_ptr<T>(m_t, m_off); return m_t; }
+	T* operator++(int) { T* tmp = m_t; operator++(); return tmp; }
+
+	bool operator==(T* p) { return p == m_t; }
+	bool operator!=(T* p) { return p != m_t; }
+	bool operator==(offset_iterator<T> &i) { return i.m_t == m_t; }
+	bool operator!=(offset_iterator<T> &i) { return i.m_t != m_t; }
+
+	T& operator*() const { return *m_t; }
+	operator T& () const { return *m_t; }
+	T* operator->() const { return m_t; }
+
+private:
+	T* m_t;
+	size_t m_off;
+};