towards "boot.rs"

[hawkw]

Now that naked functions are a thing, we can potentially start transitioning some code out of the asm boot files into Rust-land.

This isn't terribly important - the current asm works fine.

Requires:

• Implement Multiboot header in pure Rust (Implemented Multiboot 2 header in pure Rust #48)
• Implement page-table mapping in pure Rust (Implement page table mapping in Rust #67)
• Set control register flags from Rust (Set control register flags from a .rs file #68)
• Figure out how to get Cargo to build & link the 32-bit boot module (Figure out how to get cargo to build and link the 32-bit boot module #69)