refactor(repository): added fs.DirectoryIterator (#3365)

* refactor(repository): added fs.DirectoryIterator

This significantly reduces number of small allocations while
taking snapshots of lots of files, which leads to faster snapshots.

```
$ runbench --kopia-exe ~/go/bin/kopia \
   --compare-to-exe ~/go/bin/kopia-baseline --min-duration 30s \
   ./snapshot-linux-parallel-4.sh
DIFF duration: current:5.1 baseline:5.8 change:-13.0 %
DIFF repo_size: current:1081614127.6 baseline:1081615302.8 change:-0.0 %
DIFF num_files: current:60.0 baseline:60.0 change:0%
DIFF avg_heap_objects: current:4802666.0 baseline:4905741.8 change:-2.1 %
DIFF avg_heap_bytes: current:737397275.2 baseline:715263289.6 change:+3.1 %
DIFF avg_ram: current:215.0 baseline:211.5 change:+1.6 %
DIFF max_ram: current:294.8 baseline:311.4 change:-5.3 %
DIFF avg_cpu: current:167.3 baseline:145.3 change:+15.1 %
DIFF max_cpu: current:227.2 baseline:251.0 change:-9.5 %
```

* changed `Next()` API

* mechanical move of the iterator to its own file

* clarified comment

* pr feedback

* mechanical move of all localfs dependencies on os.FileInfo to a separate file

* Update fs/entry.go

Co-authored-by: ashmrtn <3891298+ashmrtn@users.noreply.github.com>

* Update fs/entry_dir_iterator.go

Co-authored-by: Julio Lopez <1953782+julio-lopez@users.noreply.github.com>

* doc: clarified valid results from Next()

---------

Co-authored-by: ashmrtn <3891298+ashmrtn@users.noreply.github.com>
Co-authored-by: Julio Lopez <1953782+julio-lopez@users.noreply.github.com>

internal/diff/diff_test.go

@@ -45,16 +45,10 @@ type testDirectory struct {
 	modtime time.Time
 }
 
-func (d *testDirectory) IterateEntries(ctx context.Context, cb func(context.Context, fs.Entry) error) error {
-	for _, file := range d.files {
-		err := cb(ctx, file)
-		if err != nil {
-			return err
-		}
-	}
-
-	return nil
+func (d *testDirectory) Iterate(ctx context.Context) (fs.DirectoryIterator, error) {
+	return fs.StaticIterator(d.files, nil), nil
 }
+
 func (d *testDirectory) SupportsMultipleIterations() bool { return false }
 func (d *testDirectory) IsDir() bool                      { return true }
 func (d *testDirectory) LocalFilesystemPath() string      { return d.name }