Define a new Features() method for Fs

Optional interfaces are becoming more important in rclone,
--track-renames and --backup-dir both rely on them.

Up to this point rclone has used interface upgrades to define optional
behaviour on Fs objects.  However when one Fs object wraps another it
is very difficult for this scheme to work accurately.  rclone has
relied on specific error messages being returned when the interface
isn't supported - this is unsatisfactory because it means you have to
call the interface to see whether it is supported.

This change enables accurate detection of optional interfaces by use
of a Features struct as returned by an obligatory Fs.Features()
method.  The Features struct contains flags and function pointers
which can be tested against nil to see whether they can be used.

As a result crypt and hubic can accurately reflect the capabilities of
the underlying Fs they are wrapping.

hubic/hubic.go

@@ -76,6 +76,7 @@ type credentials struct {
 // Fs represents a remote hubic
 type Fs struct {
 	fs.Fs                    // wrapped Fs
+	features    *fs.Features // optional features
 	client      *http.Client // client for oauth api
 	credentials credentials  // returned from the Hubic API
 	expires     time.Time    // time credentials expire
@@ -169,37 +170,13 @@ func NewFs(name, root string) (fs.Fs, error) {
 		return nil, err
 	}
 	f.Fs = swiftFs
+	f.features = f.Fs.Features().Wrap(f)
 	return f, err
 }
 
-// Purge deletes all the files and the container
-//
-// Optional interface: Only implement this if you have a way of
-// deleting all the files quicker than just running Remove() on the
-// result of List()
-func (f *Fs) Purge() error {
-	fPurge, ok := f.Fs.(fs.Purger)
-	if !ok {
-		return fs.ErrorCantPurge
-	}
-	return fPurge.Purge()
-}
-
-// Copy src to this remote using server side copy operations.
-//
-// This is stored with the remote path given
-//
-// It returns the destination Object and a possible error
-//
-// Will only be called if src.Fs().Name() == f.Name()
-//
-// If it isn't possible then return fs.ErrorCantCopy
-func (f *Fs) Copy(src fs.Object, remote string) (fs.Object, error) {
-	fCopy, ok := f.Fs.(fs.Copier)
-	if !ok {
-		return nil, fs.ErrorCantCopy
-	}
-	return fCopy.Copy(src, remote)
+// Features returns the optional features of this Fs
+func (f *Fs) Features() *fs.Features {
+	return f.features
 }
 
 // UnWrap returns the Fs that this Fs is wrapping
@@ -207,16 +184,8 @@ func (f *Fs) UnWrap() fs.Fs {
 	return f.Fs
 }
 
-// Hashes returns the supported hash sets.
-// Inherited from swift
-func (f *Fs) Hashes() fs.HashSet {
-	return fs.HashSet(fs.HashMD5)
-}
-
 // Check the interfaces are satisfied
 var (
 	_ fs.Fs        = (*Fs)(nil)
-	_ fs.Purger    = (*Fs)(nil)
-	_ fs.Copier    = (*Fs)(nil)
 	_ fs.UnWrapper = (*Fs)(nil)
 )