Add section with alloc only but not libstd

[johnthagen]

When rust-lang/rfcs#2480 is stabilized and implemented, it should be possible to create binaries that don't depend on std but do depend on alloc. This would be a step between no_std and no_main, because at least you could use the heap. The actual sample hello world code would probably not change much. It would probably be best to show off using a Vec in this instance.

This could probably be added now as a nightly subproject.