stim: re-express the AbstractStimulus list op as a fold

this gives a big perf boost: 10.5 fps -> 446 fps.

still far lower from the 720 fps we got on an ordinary Vec<Box<dyn
AbstractRegion>>. i think we had achieved 730  using the old
ListVisitor.

it's probably not worth list-ifying the stimuli; at least not at this
level. at the least, we probably want only 5 stimuli: one per core.
if we did that, the stimuli could even have all the same typename,
and be put into a plain old array; no boxing.

crates/coremem/src/stim.rs

@@ -1,7 +1,7 @@
 use crate::real::Real as _;
 use crate::cross::vec::Vec3;
 use crate::geom::{HasCrossSection, Meters, Region};
-use coremem_cross::compound::list::{Map, MapVisitor, Sum};
+use coremem_cross::compound::list::{Fold, FoldOp};
 use rand;
 
 /// field densities
@@ -45,21 +45,19 @@ struct StimulusEvaluator {
     pos: Meters,
 }
 
-impl<S: AbstractStimulus> MapVisitor<&S> for StimulusEvaluator {
+impl<S: AbstractStimulus> FoldOp<Fields, &S> for StimulusEvaluator {
     type Output = Fields;
-    fn map(&self, s: &S) -> Self::Output {
-        s.at(self.t_sec, self.pos)
+    fn feed(&self, prev: Fields, next: &S) -> Self::Output {
+        prev + next.at(self.t_sec, self.pos)
     }
 }
 
 impl<L: Sync> AbstractStimulus for L
 where
-    for<'a> &'a L: Map<StimulusEvaluator>,
-    for<'a> <&'a L as Map<StimulusEvaluator>>::Output: Sum<Fields, Output=Fields>,
+    for<'a> &'a L: Fold<StimulusEvaluator, Fields, Output=Fields>,
 {
     fn at(&self, t_sec: f32, pos: Meters) -> Fields {
-        self.map(StimulusEvaluator { t_sec, pos })
-            .sum(Fields::default())
+        self.fold(StimulusEvaluator { t_sec, pos}, Fields::default())
     }
 }