Now, the return type of functions spawned with `Group.async` and
`Group.concurrent` may be anything that coerces to `Io.Cancelable!void`.

Before this commit, group tasks were the only exception to the rule
"error.Canceled should never be swallowed". Now, there is no exception,
and it is enforced with an assertion upon closure completion.

Finally, fixes a case of swallowing error.Canceled in the compiler,
solving a TODO.

There are three ways to handle `error.Canceled`. In order of most
common:
1. Propagate it
2. After receiving it, io.recancel() and then don't propagate it
3. Make it unreachable with io.swapCancelProtection()