benchmark: add a comparison of 'archive/tar'
benchmark: add a comparison of 'archive/tar'

Since this project has forked logic of upstream 'archive/tar', this does
a brief comparison including the RawBytes usage.

```bash
$ go test -run="XXX" -bench=.
testing: warning: no tests to run
BenchmarkUpstreamTar-4                      2000            700809 ns/op
BenchmarkOurTarNoAccounting-4               2000            692055 ns/op
BenchmarkOurTarYesAccounting-4              2000            723184 ns/op
PASS
ok      vb/tar-split    4.461s
```

From this, the difference is negligible.

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

vbatts/tar-split