Add backend.ReaderAt

src/restic/backend/readerat.go

@@ -0,0 +1,19 @@
+package backend
+
+import (
+	"io"
+)
+
+type backendReaderAt struct {
+	be Backend
+	h  Handle
+}
+
+func (brd backendReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
+	return brd.be.Load(brd.h, p, off)
+}
+
+// ReaderAt returns an io.ReaderAt for a file in the backend.
+func ReaderAt(be Backend, h Handle) io.ReaderAt {
+	return backendReaderAt{be: be, h: h}
+}