[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/boot/allocator.h

@@ -0,0 +1,57 @@
+#pragma once
+/// \file allocator.h
+/// Page allocator class definition
+
+#include "kernel_args.h"
+
+namespace uefi {
+	class boot_services;
+}
+
+namespace boot {
+namespace memory {
+
+using alloc_type = kernel::init::allocation_type;
+
+class allocator
+{
+public:
+	using allocation_register = kernel::init::allocation_register;
+
+	allocator(uefi::boot_services &bs);
+
+	void * allocate(size_t size, bool zero = false);
+	void free(void *p);
+
+	void * allocate_pages(size_t count, alloc_type type, bool zero = false);
+
+	void memset(void *start, size_t size, uint8_t value);
+	void copy(void *to, void *from, size_t size);
+
+	inline void zero(void *start, size_t size) { memset(start, size, 0); }
+
+	allocation_register * get_register() { return m_register; }
+
+private:
+	void add_register();
+
+	uefi::boot_services &m_bs;
+
+	allocation_register *m_register;
+	allocation_register *m_current;
+};
+
+/// Initialize the global allocator
+void init_allocator(uefi::boot_services *bs);
+
+} // namespace memory
+
+extern memory::allocator &g_alloc;
+
+} // namespace boot
+
+void * operator new (size_t size, void *p);
+void * operator new(size_t size);
+void * operator new [] (size_t size);
+void operator delete (void *p) noexcept;
+void operator delete [] (void *p) noexcept;