Waterfall hooks to accept receiver as return type

[sugarmanz]

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

Currently, you'll get a validation error for the following:

@SyncWaterfall<A.(b: B) -> 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.