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/funcs.go

@@ -19,6 +19,7 @@
 package funcs
 
 import (
+	"context"
 	"fmt"
 	"strings"
 	"sync"
@@ -150,6 +151,8 @@ func PureFuncExec(handle interfaces.Func, args []types.Value) (types.Value, erro
 	hostname := ""                                   // XXX: add to interface
 	debug := false                                   // XXX: add to interface
 	logf := func(format string, v ...interface{}) {} // XXX: add to interface
+	ctx, cancel := context.WithCancel(context.TODO())
+	defer cancel()
 
 	info := handle.Info()
 	if !info.Pure {
@@ -217,7 +220,7 @@ func PureFuncExec(handle interfaces.Func, args []types.Value) (types.Value, erro
 		if debug {
 			logf("Running func")
 		}
-		err := handle.Stream() // sends to output chan
+		err := handle.Stream(ctx) // sends to output chan
 		if debug {
 			logf("Exiting func")
 		}
@@ -296,10 +299,7 @@ Loop:
 		}
 	}
 
-	if err := handle.Close(); err != nil {
-		err = errwrap.Append(err, reterr)
-		return nil, errwrap.Wrapf(err, "problem closing func")
-	}
+	cancel()
 
 	if result == nil && reterr == nil {
 		// programming error