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.