lang: interfaces, funcs: Implement fmt.Stringer for functions

This adds the requirement that all function implementations provider a
String() string method so that these can be used as vertices in the
pgraph library. If we eventually move to generics for the pgraph DAG,
then this might not matter, but it's not bad that these have names
either.

lang/funcs/operator_polyfunc.go

@@ -435,6 +435,13 @@ type OperatorPolyFunc struct {
 	closeChan chan struct{}
 }
 
+// String returns a simple name for this function. This is needed so this struct
+// can satisfy the pgraph.Vertex interface.
+func (obj *OperatorPolyFunc) String() string {
+	// TODO: return the exact operator if we can guarantee it doesn't change
+	return OperatorFuncName
+}
+
 // argNames returns the maximum list of possible argNames. This can be truncated
 // if needed. The first arg name is the operator.
 func (obj *OperatorPolyFunc) argNames() ([]string, error) {