Backend refactor

[LilithSilver]

Refactors the backend to use futures instead of signals, and to use multiple separate modules instead of one giant driver.

TODO before this PR is out of draft:

• Test local and remote servers
• Figure out binary doc loading
• Implement merge
• Implement revert
• Delete branch
• Make branch list work
• with_document breaks if repo.stop() is called in the middle
• Server status info thingy
• Write the config file
• Checkout saved branch on startup
• Fix FS watcher not actually using ignore globs (Nikita)
• Fix deadlock with initial loading projects
• Make diffs work again (Nikita)
  • Figure out how to send Variants across threads, or alternatively figure out how to create diffs without using Variants
• Finish the Variant parser
• Figure out loading bars? Probably stretch
• Support stable tokio and no console in Release mode
• Clean up TODOs

Performance/correctness TODOs:

• See if we can figure out why get_changes is so slow, and why it causes a freeze even on a different thread (and make it better)
• Validate the concurrency of main_thread_block.rs with a second pair of eyes
• See if I can reduce block_on for getters on the VM