pub fn default_run(&self) -> Option<&'g str> {
        self.inner.default_run.as_ref().map(|x| x.as_ref())
    }
Returns the binary that is run by default, if specified.

Information about this binary can be queried using [the `build_target`
method](Self::build_target).

This is derived from the `default-run` field of `Cargo.toml`.
guppy/src/graph/graph_impl.rs