CLI should not depend on rustup

[bfops]

Phoebe: A small but meaningful subset of users (myself included) use Rust without rustup. Currently, this is painful, because the CLI unconditionally invokes rustup to ensure the wasm32-unknown-unknown target is installed. I ended up having to put a stub shell script into my path as rustup which does nothing and returns 0.

The CLI should verify that cargo has the wasm32-unknown-unknown target installed, and if it is present, not invoke rustup.

Bonus points if the CLI checks for rustup on the system when wasm32-unknown-unknown is not present, and if neither the target nor rustup are present, prints an error message telling the user to install the target.