style: factor the event-pair type out of the reconvergence fixture

clippy's `type_complexity` fires on the tuple-of-vectors return. Caught
after pushing, because the verification chain used `&&` between `just
lint` and a `grep` that succeeded on the error text — so a failing lint
reported success. The gate has to be on the command, not on whether the
output matched.

tests/reconvergence_equivalence.rs

@@ -34,7 +34,7 @@ fn ev(a: &str, b: &str, time: i64) -> Event<i64, String> {
 }
 
 /// Ingest each chunk in turn, converging fully after every one.
-fn fit_in_chunks(chunks: Vec<Vec<Event<i64, String>>>) -> Vec<(String, Gaussian)> {
+fn fit_in_chunks(chunks: Vec<Events>) -> Vec<(String, Gaussian)> {
     let mut h: History<i64, _, _, String> =
         History::builder_with_key().convergence(tight()).build();
 
@@ -76,7 +76,10 @@ fn assert_same(a: &[(String, Gaussian)], b: &[(String, Gaussian)], what: &str) {
     }
 }
 
-fn fixture() -> (Vec<Event<i64, String>>, Vec<Event<i64, String>>) {
+type Events = Vec<Event<i64, String>>;
+
+/// Two chunks of events: the first at times 0..20, the second at 100..120.
+fn fixture() -> (Events, Events) {
     let names = ["a", "b", "c", "d", "e"];
     let mut seed = 7u64;
     let mut rnd = move || {