Sources and sinks must have unique names

[mrwalker]

Currently sources and sinks must have unique names because they are stored as name -> tap maps. This results in a branch/pass anti-pattern when people want to write the output of an assembly to different taps. It can also be surprising when earlier sinks are compiled away by c.j.

We should exception on duplicate source/sink names, but maybe also provide an optional parameter to distinguish between the name of the tap and the assembly from which it reads.