Add send_all for pipes

[eholk]

This might also be called broadcast.

Part of #1255.

[bblum]

I'm not sure I follow what this would mean -- pipes are 1-1, so either this means sending on multiple pipes (which can be done with a vector iteration and doesn't need to be a primitive), or sending on a pipe whose other endpoint is a SharedChan. Conceivably this could mean iterating a try_send until it fails, but there's no way for a Port to know if its Chan is shared or not, let alone atomically make sure a woken receiver doesn't receive again before the broadcast is finished.

[eholk]

If I recall correctly, send_all is "send a copy of this value to every port in this list."

[bblum]

ok, I'm gonna tag this and its friends as E-easy.

[thestinger]

Removed by #8170, the bug tracking a rewrite is #7668.

[bblum]

As #3018, not actually relevant to the protocol compiler

[pnkfelix]

Accepting P-low

[TyOverby]

Should this be closed due to #7668?

[tomjakubowski]

Is this still open because a broadcast function is still needed in std::comm? I've got a patch almost ready with broadcast and broadcast_opt.

[alexcrichton]

The initial impetus for these issues has long since become outdated. The std::comm implementation has changed many times in the meantime. @tomjakubowski for now I'm going to close this (and the related two issues) as not being necessary for now. We can continue to consider if these would be necessary for today's channels, but theses issues are dated enough that we're not getting lots of useful information from them any more.