Normative: To use await in a pipeline, do x |> await f

[littledan]

Subsumes #66

[littledan]

I discussed this issue with @tschneidereit and others at the November 2017 TC39, who underscored that it was really important to have a solution for await and pipeline built-in, rather than banning the intersection. The feeling I got from some committee members (sorry, I don't remember which ones) was that it was OK to have the difference in semantics between |> await f and |> (await f). So, as a start, I'll merge this option.