feat(x86_64): const identity map first 1 GiB in Rust

[mkroening]

This PR

1. removes the unused second level-1 page table,
2. avoids panicking when a page is mapped that is already mapped,
3. identity-maps the first 2 MiB of memory using 4 KiB pages at assembly time, avoiding runtime page table initialization,
4. makes the paging code use identity-mapped page tables,
5. removes the recursive page table entry,
6. identity-maps the first 1 GiB of memory using 2 MiB pages at assembly time, which is supported by all x86-64 CPUs,
7. migrates the page table structures and definitions to Rust.

This depends on:

• test: drop hello_c and update x86_64 images #567
• refactor(x86_64/entry): make page tables readable, local, and debuggable #568