Added multicaster stream

[EricMCornelius]

An abstraction for reading from a single input and piping to various outputs

[s-ludwig]

Just one comment regarding the constructor: The outputs argument should be .duped, because at least on older versions of DMD, something like new MultiplexerStream(stream1, stream2) would silently allocate the outputs array on the stack and thus leads to memory corruption later on. Maybe this got fixed recently, but at least DMD 2.064 still has to be supported right now. Otherwise looks good to merge.

[etcimon]

I think you're suggesting that this would be a demultiplexer, because multiplexing involves merging multiple streams into one. This seems to be more like stream broadcaster. I may be wrong about this, but the principle I've understood about a multiplexer is that it should write to one of many outputs exclusively depending on a predicate rather than do a raw copy of the entire stream to each sink.

[s-ludwig]

@etcimon You are right of course. I think the most appropriate name would be MulticastStream, because broadcasting would mean to somehow send to all streams. Such a concept could maybe be implemented using a special BroadcastReceiverStream along with a channel ID, but that's a different thing.

@EricMCornelius Sorry for debating so much on details, but this is important to get right, so that we don't have to possibly break code later.