Allow any closure to be passed as an executor

[thomaseizinger]

Previously, only dedicated implementations of Executor could be passed to set_executor of NetworkConfig. To pass a closure, one had to use executor_fn. Unfortunately, that function isn't exposed on the SwarmBuilder and hence cannot be used from the outside.

With this patch, any closure that takes a future can be passed as an executor.

This allows us to:

• simplify the build function of NetworkConfig by removing the SpawnImpl
• use functions like tokio::spawn or async_std::task::spawn as the executor on the SwarmBuilder because they share the same interface

I believe this is a breaking change because we have to remove certain impl blocks from libp2p-core.

[mxinden]

This looks good to me.

I am not sure whether we depend on the & T or &mut T impl anywhere.

In case no one else knows I will take another look.