Use tokio_tar instead of async_tar (#1170)

## Summary

`tokio_tar` is a fork of `async_tar` that uses Tokio instead of
`async-std`. Using it removes a significant dependency from our tree.

(There is an open PR
(dignifiedquire/async-tar#41) in `async-tar` to
add Tokio support, but it's over a year old.)

See:
#1157 (comment).