walk: Implement walk.ListR which will use ListR if at all possible

It otherwise has the nearly the same interface as walk.Walk which it
will fall back to if it can't use ListR.

Using walk.ListR will speed up file system operations by default and
use much less memory and start immediately compared to if --fast-list
had been supplied.

fstest/fstests/fstests.go

@@ -782,6 +782,39 @@ func Run(t *testing.T, opt *Opt) {
 				TestFsListDirFile2(t)
 			})
 
+			// Test the files are all there with walk.ListR recursive listings
+			t.Run("FsListR", func(t *testing.T) {
+				skipIfNotOk(t)
+				objs, dirs, err := walk.GetAll(remote, "", true, -1)
+				require.NoError(t, err)
+				assert.Equal(t, []string{
+					"hello_ sausage",
+					"hello_ sausage/êé",
+					"hello_ sausage/êé/Hello, 世界",
+					"hello_ sausage/êé/Hello, 世界/ _ ' @ _ _ & _ + ≠",
+				}, dirsToNames(dirs))
+				assert.Equal(t, []string{
+					"file name.txt",
+					"hello_ sausage/êé/Hello, 世界/ _ ' @ _ _ & _ + ≠/z.txt",
+				}, objsToNames(objs))
+			})
+
+			// Test the files are all there with
+			// walk.ListR recursive listings on a sub dir
+			t.Run("FsListRSubdir", func(t *testing.T) {
+				skipIfNotOk(t)
+				objs, dirs, err := walk.GetAll(remote, path.Dir(path.Dir(path.Dir(path.Dir(file2.Path)))), true, -1)
+				require.NoError(t, err)
+				assert.Equal(t, []string{
+					"hello_ sausage/êé",
+					"hello_ sausage/êé/Hello, 世界",
+					"hello_ sausage/êé/Hello, 世界/ _ ' @ _ _ & _ + ≠",
+				}, dirsToNames(dirs))
+				assert.Equal(t, []string{
+					"hello_ sausage/êé/Hello, 世界/ _ ' @ _ _ & _ + ≠/z.txt",
+				}, objsToNames(objs))
+			})
+
 			// TestFsListDirRoot tests that DirList works in the root
 			TestFsListDirRoot := func(t *testing.T) {
 				skipIfNotOk(t)