union: implement missing methods

Implement these missing methods:

- CleanUp

And declare these ones unimplementable:

- UnWrap
- WrapFs
- SetWrapper
- UserInfo
- Disconnect
- PublicLink
- PutUnchecked
- MergeDirs
- OpenWriterAt

backend/union/union.go

@@ -801,6 +801,24 @@ func (f *Fs) Shutdown(ctx context.Context) error {
 	return errs.Err()
 }
 
+// CleanUp the trash in the Fs
+//
+// Implement this if you have a way of emptying the trash or
+// otherwise cleaning up old versions of files.
+func (f *Fs) CleanUp(ctx context.Context) error {
+	errs := Errors(make([]error, len(f.upstreams)))
+	multithread(len(f.upstreams), func(i int) {
+		u := f.upstreams[i]
+		if do := u.Features().CleanUp; do != nil {
+			err := do(ctx)
+			if err != nil {
+				errs[i] = fmt.Errorf("%s: %w", u.Name(), err)
+			}
+		}
+	})
+	return errs.Err()
+}
+
 // NewFs constructs an Fs from the path.
 //
 // The returned Fs is the actual Fs, referenced by remote in the config
@@ -963,4 +981,5 @@ var (
 	_ fs.Abouter         = (*Fs)(nil)
 	_ fs.ListRer         = (*Fs)(nil)
 	_ fs.Shutdowner      = (*Fs)(nil)
+	_ fs.CleanUpper      = (*Fs)(nil)
 )