Moves the code which executes subcommands into Subcommand:run,
delegating to separate functions for each subcommand. This reduces the
disgustingness of `run::run` a bit, and paves the way for future
refactoring and cleanup.
- Differentiate between `arg`s, which are flags and options, and `cmd`s,
which are mutually exclusive subcommands
- Replace string literals, like "EVALUATE", with constants, like
`cmd::EVALUATE`, since they're slightly less error prone.
- Remove `Config::evaluate`, and handle it like other subcommands