Add `cargo run --example` and `cargo run --bin`

This lets users run any executable from the `examples` or `bin` directories by
passing its name with `--example` or `--bin` flag.

If neither is specified, we fall back to the old behaviour (running the only bin
target in the project, failing if there are more).

Closes #538
Instead of recalculating where the test specified was built, could this add some extra metadata to the `Compilation` structure saying where each named target went? I'd rather use the result of `compile.tests` here to reduce duplication of where this is calculated