Retry naked functions

[mkroening]

Naked functions have come a long way since #95.

There is rust-lang/rfcs#2972 and rust-lang/rust#79653 which help against misusing the feature. We have adapted it in eduOS successfully, making it possible to use const and sym operands in asm! blocks instead of having to resort to global symbols. We could also eliminate most, if not all, hard coded constants from assembly, making the code much easier to understand.

What do you think, @stlankes?