storage: add 'Has' feature.

I debated for a while if there's any way to avoid doing interfaces that
refer to other interfaces here, because I usually find that's a smell,
but any idea I can think of to try to avoid it seems worse (e.g.
would threaten to spawn a 'Has(R)' *and* a 'Has2(W)' function on the
package scope, due to golang's lack of polymorphic parameters, etc).

storage/api.go

@@ -7,11 +7,17 @@ import (
 
 // --- basics --->
 
+type Storage interface {
+	Has(ctx context.Context, key string) (bool, error)
+}
+
 type ReadableStorage interface {
+	Storage
 	Get(ctx context.Context, key string) ([]byte, error)
 }
 
 type WritableStorage interface {
+	Storage
 	Put(ctx context.Context, key string, content []byte) error
 }
 

storage/funcs.go

@@ -17,6 +17,11 @@ import (
 	regardless of how much explicit support the storage implementation has for the exact behavior you requested.
 */
 
+func Has(ctx context.Context, store Storage, key string) (bool, error) {
+	// Okay, not much going on here -- this function is only here for consistency of style.
+	return store.Has(ctx, key)
+}
+
 func Get(ctx context.Context, store ReadableStorage, key string) ([]byte, error) {
 	// Okay, not much going on here -- this function is only here for consistency of style.
 	return store.Get(ctx, key)