Tweak the command verbs and establish 'synonyms'
Tweak the command verbs and establish 'synonyms'

This reorganizes the verbs used in rustup commands. The concept here
is that every subcommand uses the most appropriate 'add/remove' verb
pair for itself, but implements hidden alternate commands using
the obvious other verbs.

So now the problematic verbs are:

* `toolchain install` / `toolchain uninstall`
* `target add` / `target remove`
* `override set` / `override unset`

Additional (hidden) synonyms are:

* `toolchain update`
* `toolchain add` / `toolchain remove`
* `target add` / `target remove`

This leaves the top level `rustup update` as the only non-hidden
'update' command, which has the unique focus of updating everything.

rust-lang/rustup,Boddlnagg/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,polonez/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,polonez/rustup.rs,polonez/rustup.rs,theindigamer/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,theindigamer/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,theindigamer/rustup.rs,rust-lang/rustup,Boddlnagg/rustup.rs,inejge/rustup.rs,Boddlnagg/rustup.rs,theindigamer/rustup.rs,polonez/rustup.rs,theindigamer/rustup.rs,Boddlnagg/rustup.rs,polonez/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,Boddlnagg/rustup.rs