Upgrade "arc download" to Toolsets

Summary:
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