Make debug builds more usable

This pr makes debug builds more usable in terms of `cargo run -- --dev`.

1. `--dev` activates `--execution native`, iff `--execution` is not
given or no sub `--execution-*` is given.
2. It was probably a mistake to compile WASM in debug for a debug build.
So, we now build the WASM binary always as `release` (if not requested
differently by the user). So, we trade compilation time for a better
debug experience.