Ignored args not properly ignored

Specifically, arguments with names beginning with `_` are not properly ignored, *if* they are bound within `awaitArg`. This is a very minor performance issue, but also allows for very confusing code such as:
```
multiply = \ m n _foo . m ( n _bar )
```
to be valid, as both `_foo` and `_bar` are not ignored, and evaluated as simply `_` so refer to the same variable.