change `run` cmd to `build + run`

Currently the `Run` button invokes the `cargo run` command which both compiles the program AND runs it.

I think it would be better to have the `Run` button:
1. invoke `cargo build`,
1. capture any compiler messages and send them to the front end,
1. THEN invoke `cargo run` and send its output to the front end

I'm just worried that if somebody's cargo project prints out JSON data, we won't be able to tell that apart from the compiler messages.

This change will be implemented entirely on the server-side. Once this is done, we can think about better ways to display the program's runtime output on the front end.