generalize List.reduce

Now does both N-ary reducers with left or right associativity, and
all the standard reducers for binary functions of any associativity.

src/core/List.pm

@@ -608,13 +608,24 @@ my class List does Positional { # declared in BOOTSTRAP
     }
 
     method reduce(List: &with) {
-        fail('can only reduce with arity 2')
-            unless &with.arity <= 2 <= &with.count;
         return unless self.DEFINITE;
-        my \vals = self.values;
-        my Mu $val = vals.shift;
-        $val = with($val, $_) for vals;
-        $val;
+        return self.values if self.elems < 2;
+        if &with.count > 2 and &with.count < Inf {
+            my $moar = &with.count - 1;
+            my \vals = self.values;
+            if try &with.prec<assoc> eq 'right' {
+                my Mu $val = vals.pop;
+                $val = with(|vals.splice(*-$moar,$moar), $val) while vals >= $moar;
+                return $val;
+            }
+            else {
+                my Mu $val = vals.shift;
+                $val = with($val, |vals.splice(0,$moar)) while vals >= $moar;
+                return $val;
+            }
+        }
+        my $reducer = find-reducer-for-op(&with);
+        $reducer(&with)(self) if $reducer;
     }
 
     method sink() {

src/core/LoL.pm

@@ -41,7 +41,8 @@ class LoL { # declared in BOOTSTRAP
 sub lol (**@l) { @l }
 
 sub find-reducer-for-op($op) {
-    my %prec := $op.prec;
+    try my %prec := $op.prec;
+    return &METAOP_REDUCE_LEFT unless %prec;
     my $reducer = %prec<prec> eq 'f='
         ?? 'listinfix'
         !! %prec<assoc> || 'left';