Upgrade "arc download" to Toolsets
Ref T13490. This is mostly straightforward.
- Drop "--show" in favor of "--as -".
- Drop support for 4+ year old "file.info" API.
- Use modern stream-to-disk support so we get a real progress bar and don't need to buffer files into memory.
Test Plan: Downloaded various files, including large files.
Maniphest Tasks: T13490
Differential Revision: https://secure.phabricator.com/D21097