fix(types): do not use arrow syntax for _start

[joemphilips]

using arrow syntax for those methods which will be overridden by interface extention sometimes cause compile error. Fix by stop using arrow syntax

When I tried to use xstream in my library, it caused error like this

../../node_modules/xstream/index.d.ts:15:18 - error TS2430: Interface 'Operator<T, R>' incorrectly extends interface 'InternalProducer<R>'.
  Types of property '_start' are incompatible.
    Type '(out: Stream<R>) => void' is not assignable to type '(listener: InternalListener<R>) => void'.
      Types of parameters 'out' and 'listener' are incompatible.
        Type 'InternalListener<R>' is not assignable to type 'Stream<R>'.
          Property '_prod' is missing in type 'InternalListener<R>'.

15 export interface Operator<T, R> extends InternalProducer<R>, InternalListener<T>, OutSender<R> {
                    ~~~~~~~~

../../node_modules/xstream/index.d.ts:20:18 - error TS2430: Interface 'Aggregator<T, U>' incorrectly extends interface 'InternalProducer<U>'.
  Types of property '_start' are incompatible.
    Type '(out: Stream<U>) => void' is not assignable to type '(listener: InternalListener<U>) => void'.
      Types of parameters 'out' and 'listener' are incompatible.
        Type 'InternalListener<U>' is not assignable to type 'Stream<U>'.

20 export interface Aggregator<T, U> extends InternalProducer<U>, OutSender<U> {

This is because interface extension have different behavior when using arrow syntax or not.

• refs: https://github.com/Microsoft/TypeScript/issues/4462

Actually, it is weird that xstream itself do not cause compile error when I run yarn compile . But anyway, This change allowed me to avoid compile error in my library.

[staltz]

Thanks! I also saw this error but had no idea how to fix it. It surprises me there is this difference with arrow function