Add support for multiple hash types.

Add support for multiple hash types with negotiation of common hash types for comparison.

Manually rebased version of #277 (see discussion there)

drive/drive.go

@@ -782,6 +782,11 @@ func (f *Fs) DirMove(src fs.Fs) error {
 	return nil
 }
 
+// Hashes returns the supported hash sets.
+func (f *Fs) Hashes() fs.HashSet {
+	return fs.HashSet(fs.HashMD5)
+}
+
 // ------------------------------------------------------------
 
 // Fs returns the parent Fs
@@ -802,8 +807,11 @@ func (o *Object) Remote() string {
 	return o.remote
 }
 
-// Md5sum returns the Md5sum of an object returning a lowercase hex string
-func (o *Object) Md5sum() (string, error) {
+// Hash returns the Md5sum of an object returning a lowercase hex string
+func (o *Object) Hash(t fs.HashType) (string, error) {
+	if t != fs.HashMD5 {
+		return "", fs.ErrHashUnsupported
+	}
 	return o.md5sum, nil
 }