Merged rustboot in two steps

Makefile
* link rust-core (extern mod core)
* don't nest tmux
* generate dependency files
* rename files
* print output file size

ARM
* use volatile_store
* MMU (disabled)
* dynamically install exception handlers
  with volatile_store
* AEABI memset (assembly)
* documentation

Kernel
* reorganize
* add Allocator trait (borrowed from rust-core)
* implement some runtime functions
* reviewed the allocator
* kernel::heap