Symbol for _destination property

[jacoscaz]

This PR complements #63 (which should be merged first) by renaming the _destination property to a dedicated Symbol, guaranteed to be unique, so that we can never trigger property collisions when setting _destination on externally-provided source objects for which isAsyncReadable(source) === true at

AsyncIterator/asynciterator.ts

Lines 2030 to 2043 in 159ed2b

	protected _wrapSource(source: InternalSource<T>) {
	try {
	this._source = isAsyncReadable(source) ? source : fromSource(source);
	}
	catch (error) {
	scheduleTask(() => this.emit('error', error));
	return;
	}
	this._source._destination = this;
	this._source.on('end', destinationClose);
	this._source.on('error', destinationEmitError);
	this._source.on('readable', destinationSetReadable);
	this.readable = true;
	}

.

[RubenVerborgh]

Thanks, @jacoscaz, I'm happy to take it from here!