Drop dependency on `generator`

[weiznich]

We do regualary a review of the dependencies of our software. While doing that we noticed that loom depends on generator which in turn seems to contain some unsound code. For example this cast from a shared reference to a mutable reference doesn't seem to be correct. Also there are quite a few long standing issues about unsound or undefined behavior in their issue tracker. Given that it might be worth to consider dropping this dependency in favor of a better solution. If someone lays out a plan what needs to change for that I might also be able to put in some time to make this happen.

[taiki-e]

If https://github.com/Amanieu/corosensei provides the functionality we need, I think it's okay to switch to it.