Refactor/declarative clap

[danieleades]

this is a reimplementation of #150 (turned out to be easier to just do it again)

the derive-based declarative clap API is

• strongly-typed
• easier to maintain
• keeps subcommand args and logic together

this refactor also allowed me to eliminate the psuedo-inheritance of the the cli::Client

this refactor also highlighted a couple of bugs in the original cli, which are harder to spot with clap's fluent API-

• the 'upgrade from-file' command is unreachable, since the CLI doesn't support it
• the reboot flag in the "recovery default-boot" command is unused

I've not addressed either of these issues in this PR