Previously, a ConditionSet's list of choices all had to accept the same

exact type of value for matches. Callers couldn't mix-and-match

different types of condition choices. If there were two pieces of data

to work with (say, a book and an author), the list of choices all had to

accept one of those, and infer the other (if possible).
Now, ConditionSet.matches() takes keyword arguments for the values,

instead of a single positional argument for a single value. These are

compared to BaseConditionChoice.value_kwarg for each condition being

matched.
By default, choices expect a value= keyword argument. More advanced

usage might have some choices that take book= and others that take

author=, as per the example above.

Unit tests pass.