Add ELF loader

[vinc]

This PR will bring us closer to be being able to run rust binaries, but some issues are still remaining. For now we'll be able to run ELF binaries produced in the following way:

nasm dsk/src/bin/hello.s -o /tmp/hello.o -f elf64
ld -o dsk/bin/hello /tmp/hello.o -Ttext=100 -Tdata=200

Instead of raw binaries produced like so:

nasm dsk/src/bin/hello.s -o /tmp/hello -f bin

[vinc]

The previous commits should fix #249