Implement Supply.flat

Because we may want to use that a lot, and it would need to be fast (as
opposed to using .map, which would add another scope to handle for each original
value).

src/core/Supply.pm

@@ -83,6 +83,7 @@ my role Supply {
 
     method for(|c)             { SupplyOperations.for(|c) }
     method interval(|c)        { SupplyOperations.interval(|c) }
+    method flat()              { SupplyOperations.flat(self) }
     method do(&side_effect)    { SupplyOperations.do(self, &side_effect) }
     method grep(&filter)       { SupplyOperations.grep(self, &filter) }
     method map(&mapper)        { SupplyOperations.map(self, &mapper) }

src/core/SupplyOperations.pm

@@ -76,6 +76,25 @@ my class SupplyOperations is repr('Uninstantiable') {
         }
         IntervalSupply.new(:$interval, :$delay, :$scheduler)
     }
+
+    method flat(Supply $a) {
+        my class FlatSupply does Supply does PrivatePublishing {
+            has $!source;
+
+            submethod BUILD(:$!source) { }
+
+            method tap(|c) {
+                my $sub = self.Supply::tap(|c);
+                $!source.tap( -> \val {
+                      self!more(val.flat)
+                  },
+                  done => { self!done(); },
+                  quit => -> $ex { self!quit($ex) });
+                $sub
+            }
+        }
+        FlatSupply.new(:source($a))
+    }
 
     method do($a, &side_effect) {
         on -> $res {