rename head_step → base_step

"head reduction" has a specific meaning in type theory and it doesn't match our head_step. So we should pick a different term. In Derek's and my class we call this "base reduction" so I figured we might as well use that terminology.

I did not rename prim_step. That term isn't standard either AFAIK, but at least it does not collide with a standard term. If we wanted to rename it we could call it thread_step, but I am not sure yet if it's worth the rename.