This reverts commit d9ca5f97fb.
There is a problem with coroutines that I haven't solved yet. Reverting
makes it work, for now.
Also, use a better coroutines flag for the LLVM opt tool.
This should make it much easier (in the future) to use the standard
library when unused functions contain unimplemented features. But more
importantly, it makes later passes better and makes compiling faster by
not having to scan dead code.
