Provide a run light for Edwin, which shows "eval" when Edwin is

Provide a run light for Edwin, which shows "eval" when Edwin is
evaluating an expression provided by the user.  This is accomplished
by setting the Edwin variable RUN-LIGHT to "eval" or false.  The
default value of MODE-LINE-PROCESS will now cause ": eval" to appear
after the names of the minor modes on the mode line, e.g.:

  (Scheme Narrow)

when no evaluation is running, and

  (Scheme Narrow: eval)

when an evaluation is running.  Because this requires a DYNAMIC-WIND
at every evaluation, Edwin variable ENABLE-RUN-LIGHT? can be used to
disable this feature.