Change the type of Process0Syntax.self to Process0

This is an attempt to fix #281. I still have to skip over `Await`s when pattern matching on `cur.step.head` in `toVector`, so I'm unsure if this is what @pchiusano had in mind. But it works and all tests pass.

Also note that I had to change one test about `id` because scalac inferred `Process[Env[Any,Any]#Is,Int]` as type for `id |> pi`.