* Start using rust-core as a submodule: https://github.com/thestinger/rust-core/
* Update build system to check for submodules' source update if passed --update-submodules
* Rejig source layout to use rust-core as our freestanding runtime
* Resolve collision between rust-core and the rust runtime std library
* Fix up the build system to initialise rust-core as a submodule as required
* Move locore.s into .boot for the moment to get the thing running
* Use as_byte()s rather than std::str's to_bytes()
* Fix up references to our own functions