Complete / Fix multi-parameter application support

[retronym]

Currently, we have a check in the ANF transform that prevents await usage multi-parameter list methods. This isn't a fundamental limitation, we just haven't written the transform that deals with Apply(Apply(....)) holistically, and doesn't change evaluation order of the arguments.

But, the check is over-eager, and currently stops use of awaitas the receiver for such a method.

[retronym]

I think I've got this working now, just needs a little cleanup and I'll submit a PR.

https://github.com/retronym/async/tree/ticket/4-multi-param