Adds a `TypedExecutor` trait that describes how to spawn futures of a specific type. This is useful for implementing functions that are generic over an executor and wish to support both `Send` and `!Send` cases.
- Move `tokio` into its own directory. - Remove `path` dependencies. - Run tests with once with crates.io dep and once with patched dep.
Use Azure Pipelines for CI. This migrates away from Travis and Appveyor.