I mostly agree with what you wrote, but I'd like to point out
that it's probably safe to move some kinds of fibers across
threads:
If the fiber's main function is pure and its parameters have no
mutable indirection (i.e. if the function is strongly pure),
there should be no way to get data races.
Therefore I believe we could theoretically support moving such
fibers. But currently I see no way how most fibers can be made
pure, after all you want to do IO in them. Of course, we could
forego the purity requirement, but then the compiler can no
longer support us.
