We changed the descending prefix to be ~ earlier, so now we need to
adjust the tests as well.

Since - leads to structopt interpreting the component as the start of
another argument, we need to use a different one.

This does currently not work yet, as we cannot call .finish() on dyn
Aggregator. This needs to be adjusted.

However, this provides the basic infrastructure for producing sorted
output, including the required command line parsing.

With the addition of our own parser, we no longer need the FromStr
implementation for SearchField. Furthermore, it is now only used in
player::NameFilter, so the definition has been moved there. The import
in the grammar was unused as well, so it has been removed.

The easiest way to get this consistent is to implement PartialOrd & Ord
for Player. However, the implementation might not be 100% sound, as it
should be using the same fields as (Partial)Eq, but that would mean
either

1. Deriving PartialOrd/Ord, which is not possible because PlayerClass
   does not implement it (which in turn would not make sense to order)
or
2. Implementing (Partial)Eq by hand instead of deriving it, which is not
   the best either

I don't think it's an issue though, as we never put Players in any
position where it might get relevant (such as a HashMap/BTreeMap), and
we're only using it to sort them for the output.