lang: interfaces, funcs: Port Func API to new Stream signature

This removes the `Close() error` and replaces it with a more modern
Stream API that takes a context. This removes boilerplate and makes
integration with concurrent code easier. The only downside is that there
isn't an explicit cleanup step, but only one function was even using
that and it was possible to switch it to a defer in Stream.

This also renames the functions from polyfunc to just func which we
determine by API not naming.

lang/funcs/core/core_test.go

@@ -583,6 +583,9 @@ func TestLiveFuncExec0(t *testing.T) {
 			valueptrch := make(chan int)        // which Nth value are we at?
 			killTimeline := make(chan struct{}) // ask timeline to exit
 
+			ctx, cancel := context.WithCancel(context.Background())
+			defer cancel()
+
 			// wait for close signals
 			wg.Add(1)
 			go func() {
@@ -617,7 +620,7 @@ func TestLiveFuncExec0(t *testing.T) {
 				if debug {
 					logf("Running func")
 				}
-				err := handle.Stream() // sends to output chan
+				err := handle.Stream(ctx) // sends to output chan
 				t.Logf("test #%d: stream exited with: %+v", index, err)
 				if debug {
 					logf("Exiting func")
@@ -740,12 +743,8 @@ func TestLiveFuncExec0(t *testing.T) {
 				t.Logf("test #%d: timeline finished", index)
 				close(argch)
 
-				t.Logf("test #%d: running Close", index)
-				if err := handle.Close(); err != nil {
-					t.Errorf("test #%d: FAIL", index)
-					t.Errorf("test #%d: could not close func: %+v", index, err)
-					return
-				}
+				t.Logf("test #%d: running cancel", index)
+				cancel()
 			}()
 
 			// read everything