Limit Filter "universes" to invocation::filter

Universes are really an optimization of filtering tasks, so let's define
them there, and derive them from the set of conditions.  This means that
complex filters might get missed and end up doing a full task scan, but
that's probably OK.

Note that this does not fix the working-set API issues (#108 and #123).

cli/src/argparse/mod.rs

@@ -19,7 +19,7 @@ mod subcommand;
 
 pub(crate) use args::TaskId;
 pub(crate) use command::Command;
-pub(crate) use filter::{Condition, Filter, Universe};
+pub(crate) use filter::{Condition, Filter};
 pub(crate) use modification::{DescriptionMod, Modification};
 pub(crate) use subcommand::Subcommand;