storage: avoid io.NopCloser; it was relatively recently introduced.

(I think our policy is roughly "support the last two significant go
versions", and by that, we might technically be allowed to use this,
but, eh; there's minimal cost to being conservative today.)

storage/funcs.go

@@ -43,7 +43,7 @@ func GetStream(ctx context.Context, store ReadableStorage, key string) (io.ReadC
 	}
 	// Fallback to basic.
 	blob, err := store.Get(ctx, key)
-	return io.NopCloser(bytes.NewReader(blob)), err
+	return noopCloser{bytes.NewReader(blob)}, err
 }
 
 // PutStream returns an io.Writer and a WriteCommitter callback.
@@ -112,9 +112,11 @@ func Peek(ctx context.Context, store ReadableStorage, key string) ([]byte, io.Cl
 	}
 	// Fallback to basic.
 	bs, err := store.Get(ctx, key)
-	return bs, noopCloser{}, err
+	return bs, noopCloser{nil}, err
 }
 
-type noopCloser struct{}
+type noopCloser struct {
+	io.Reader
+}
 
 func (noopCloser) Close() error { return nil }

storage/memstore/memstore.go

@@ -84,7 +84,7 @@ func (store *Store) GetStream(ctx context.Context, key string) (io.ReadCloser, e
 	if !exists {
 		return nil, fmt.Errorf("404") // FIXME this needs a standard error type
 	}
-	return io.NopCloser(bytes.NewReader(content)), nil
+	return noopCloser{bytes.NewReader(content)}, nil
 }
 
 // Peek implements go-ipld-prime/storage.PeekableStorage.Peek.
@@ -93,9 +93,11 @@ func (store *Store) Peek(ctx context.Context, key string) ([]byte, io.Closer, er
 	if !exists {
 		return nil, nil, fmt.Errorf("404") // FIXME this needs a standard error type
 	}
-	return content, noopCloser{}, nil
+	return content, noopCloser{nil}, nil
 }
 
-type noopCloser struct{}
+type noopCloser struct {
+	io.Reader
+}
 
 func (noopCloser) Close() error { return nil }