communication-alt statements

[graydon]

The 'alt { ... }' is a 'simple' readiness-switch for sends/receives over
multiple channels/ports. Doesn't do heavyweight poll() or bind/unbind event
channels, just fixed number of send/receive options. Need to add this.

[graydon]

Shifted to rustc.

[marijnh]

This can be considered obsolete if we move to a scheme where receive is a library operator.