Slight optimization to //set

Just noticed I box the one type version in a list, to avoid testing
whether it's the one type version, but have to test for that to decide
whether to box it or not. Should shave like a whole 3ms from each //set
command.
CPS-ifying stack

Continuation Passing Style lets me use minetest.after, so the server
gets a chance to not hang in between every stack iteration. Could even
set minetest.after(1000,nextone) if you want to see it extend once every
second.
Merge pull request #51 from cyisfor/master

I think that's the optimization you mentioned?