Add assert_once_ever macro. Close #7748. (fixme cf #8472)
small cleanups in task/spawn.rs
Don't fail in port.try_recv() the second time. Close #7800.
Only bug on self-not-mapped-to-def if no previous errors were present. Close #6642.
auto merge of #8677 : bblum/rust/scratch, r=alexcrichton

r anybody; there isn't anything complicated here