chore: refactor responsibilities

[BrianHicks]

Currently:

• Workspace is responsible for setting up files for a job
• Job is responsible for creating the command
• Runner is responsible for bringing the workspace and job together
• Coordinator is responsible for executing jobs in the right order

It might make sense to refactor these in terms of responsibilities:

• Runner is responsible for taking a job (not a job and workspace, just a job), running it, and saying where we can get the output
  • for local builds, this probably just looks like managing the workspace
  • for remote builds, this looks like shipping the job definition and any necessary files to a remote executor, then downloading the result again (to be clear, remote builds are not part of this story!)
• Workspace is responsible for isolating the file system as much as possible
  • it symlinks files into the working directory
  • it creates a fake HOME (see isolate HOME #62)
  • it symlinks any other jobs in
• Job is responsible for command isolation
  • it isolates the PATH (see isolate binaries to only those specified in the job #61, or should Workspace maybe handle this?)
  • or maybe we should have some other kind of command that wraps std::process::Command that can do this wrapping for us?
• Coordinator is responsible for the same things as before

This'll be a biggish refactor, but should mean we have better-defined responsibilities for all the components in the system.