"humility hiffy --list" panics on some archives

[bcantrill]

When run against some archives (and, notably, a recent Hubris release), humility hiffy --list panics:

$ HUMILITY_ARCHIVE=./build-gimlet-c-dev-image-default-v1.0.2.zip humility -p archive hiffy --list
...
INTERFACE                    TASK
Sensor                       sensor
  |
  +--> Sensor.get
  |       id                          SensorId
  |       <ok>                        f32
  |       <error>                     SensorError
  |
  +--> Sensor.get_reading
  |       id                          SensorId
  |       <ok>                        Reading
  |       <error>                     SensorError
  |
  +--> Sensor.get_raw_reading
  |       id                          SensorId
thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', /home/bmc/.cargo/registry/src/github.com-1ecc6299db9ec823/multimap-0.8.3/src/lib.rs:822:45
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

As it turns out, this is due to havarnov/multimap#46. There is a fix for that issue, but we can also modify our code to avoid an iter() (and replace it with keys() -- which is what the code is trying to do anyway).