To introduce new remote-helper changes like ‘dont push by default’ and ‘self’ / ‘rad’ remotes, this crate has been refactored to ease testing.

• the command parsing has been refactored into a sans i/o esque typed parser.
• the radicle node interactions have been hoisted into a NodeSession trait.
• the git send_pack and fetch_pack interactions have hoisted into GitService trait.
• the direct stdin/out printing has been replaced with BufRead and Writer traits.

Continuation of the refactoring.

• Reorder and squash into slightly bigger but still cohesive commits.
• Reword commit messages.