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/structs/const.go

@@ -24,6 +24,11 @@ import (
 	"github.com/purpleidea/mgmt/lang/types"
 )
 
+const (
+	// ConstFuncName is the unique name identifier for this function.
+	ConstFuncName = "const"
+)
+
 // ConstFunc is a function that returns the constant value passed to Value.
 type ConstFunc struct {
 	Value types.Value
@@ -32,6 +37,12 @@ type ConstFunc 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 *ConstFunc) String() string {
+	return ConstFuncName
+}
+
 // Validate makes sure we've built our struct properly.
 func (obj *ConstFunc) Validate() error {
 	if obj.Value == nil {