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/core/datetime/now_fact.go

@@ -24,8 +24,14 @@ import (
 	"github.com/purpleidea/mgmt/lang/types"
 )
 
+const (
+	// NowFuncName is the name this fact is registered as. It's still a Func
+	// Name because this is the name space the fact is actually using.
+	NowFuncName = "now"
+)
+
 func init() {
-	facts.ModuleRegister(ModuleName, "now", func() facts.Fact { return &DateTimeFact{} }) // must register the fact and name
+	facts.ModuleRegister(ModuleName, NowFuncName, func() facts.Fact { return &DateTimeFact{} }) // must register the fact and name
 }
 
 // DateTimeFact is a fact which returns the current date and time.
@@ -34,6 +40,12 @@ type DateTimeFact struct {
 	closeChan chan struct{}
 }
 
+// String returns a simple name for this fact. This is needed so this struct can
+// satisfy the pgraph.Vertex interface.
+func (obj *DateTimeFact) String() string {
+	return NowFuncName
+}
+
 // Validate makes sure we've built our struct properly. It is usually unused for
 // normal facts that users can use directly.
 //func (obj *DateTimeFact) Validate() error {