Add Provider interface

provider.go

@@ -2,8 +2,12 @@ package step
 
 import "html/template"
 
+// Provider is an object which provides extra functionality to an environment.
+type Provider any
+
 // FuncProvider provides a template.FuncMap.
 type FuncProvider interface {
+	Provider
 	// FuncMap provides a template.FuncMap. It may return nil, in which case
 	// its result is simply not considered.
 	FuncMap () template.FuncMap
@@ -12,7 +16,7 @@ type FuncProvider interface {
 // FuncProviderFor is an object that provides a template.FuncMap for a specific
 // document.
 type FuncProviderFor interface {
-	FuncProvider
+	Provider
 	// FuncMap provides a template.FuncMap, given a particular document. It
 	// may return nil, in which case its result is simply not considered.
 	FuncMapFor (*Document) template.FuncMap
@@ -20,6 +24,7 @@ type FuncProviderFor interface {
 
 // Configurable is an object that can be configured according to metadata.
 type Configurable interface {
+	Provider
 	// Configure uses config to configure the object. It must not modify
 	// config. Keys are namespaced with a '.' and are written in kebab case.
 	Configure (config Meta) error