Remove ELF and initrd loading from the kernel. The bootloader now loads
the initial programs, as it does with the kernel. Other files that were
in the initrd are now on the ESP, and non-program files are just passed
as modules.
The page table code had been copied mostly verbatim from the kernel, and
was a dense mess. I abstraced the `page_table_indices` class and the old
loop behavior of `map_in` into a new `page_entry_iterator` class, making
both `map_pages` and the initial offset mapping code much cleaner.
Tags: vmem paging
Set up initial page tables for both the offset-mapped area and the
loaded kernel code and data.
* Got rid of the `loaded_elf` struct - the loader now runs after the
initial PML4 is created and maps the ELF sections itself.
* Copied in the `page_table` and `page_table_indices` from the kernel,
still need to clean this up and extract it into shared code.
* Added `page_table_cache` to the kernel args to pass along free pages
that can be used for initial page tables.
Tags: paging
