Waterfall hooks to accept receiver as return type

**Is your feature request related to a problem? Please describe.**

Currently, you'll get a validation error for the following:
```kotlin
@SyncWaterfall A>
val hook: Hook
```
> `Waterfall hooks must specify the same types for the first parameter and the return type`

**Describe the solution you'd like**

Receivers are just syntactic sugar for first parameters, so it'd be cool to accept the receiver as the accumulator type for a waterfall hook.