Demonstrates string traces

Demonstrates string traces

When declaring a Signal, you can pass a custom decoder that translates
the Signal logic level to a string. nMigen uses this internally to
display Enum traces, but it is available for general use.

Some applications are:
1) Display a string when a signal is at high level, otherwise show a
single horizontal line. Useful to draw attention to a time interval.
2) Display the stages of a unit test
3) Display arbitrary debug statements along the timeline.