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/os/readfile_func.go

@@ -29,8 +29,13 @@ import (
 	"github.com/purpleidea/mgmt/util/errwrap"
 )
 
+const (
+	// ReadFileFuncName is the name this function is registered as.
+	ReadFileFuncName = "readfile"
+)
+
 func init() {
-	funcs.ModuleRegister(ModuleName, "readfile", func() interfaces.Func { return &ReadFileFunc{} }) // must register the func and name
+	funcs.ModuleRegister(ModuleName, ReadFileFuncName, func() interfaces.Func { return &ReadFileFunc{} }) // must register the func and name
 }
 
 // ReadFileFunc is a function that reads the full contents from a local file. If
@@ -51,6 +56,12 @@ type ReadFileFunc 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 *ReadFileFunc) String() string {
+	return ReadFileFuncName
+}
+
 // ArgGen returns the Nth arg name for this function.
 func (obj *ReadFileFunc) ArgGen(index int) (string, error) {
 	seq := []string{"filename"}