This PR re-implements `IO.waitAny` using Lean instead of C++. This is to
reduce the size and
complexity of `task_manager` in order to ease future refactorings.
There is an import behavioral change of `IO.waitAny` in this PR.
Consider a situation where we have
two promises `p1`, `p2` and call `IO.waitAny [p1.result!, p2.result!]`
and `p1` resolves instantly.
Previously this would just return the result of `p1` and require nothing
else. With the new
implementation if `p2` is released before being resolved this can cause
a panic, even if
`IO.waitAny` has already finished. I argue that this is reasonable
behavior, given that an
invocation of `result!` promises that the promise will eventually be
resolved.
6d5ce9b87f