Make `futures-channel` and `futures-exectuor` optional

[dpc]

[robinfriedli]

Those dependencies are required by the evaluate functions and JoinHandle, which I consider a core feature. The dependencies are only not required if the async feature is enabled, because then futures could be used directly instead. However, having optional dependencies based on a feature NOT being enabled is not possible afaik: https://github.com/rust-lang/cargo/issues/1839