chore: refactor of command invocation handling

[rikonor]

This PR makes several standardizing changes:

1. Removes the various dispatch functions in favor of a centralized setup in main.
2. Removes various Command structs that were actually unused.
3. Standardizes on Subcommand enums being called Command.
4. Standardizes on exec functions taking reference to args, instead of a cmd, and renames all Cmd structs to <Command>Args.

[raymondk]

Seems fine other than the ordering of the canister commands. Maybe sorting the enum fixes it.