Add support for examples and external tests

This add support for `examples/*.rs` being built (as normal bin crates) during `cargo test`,
and `src/test.rs` and `tests/*.rs` being built and run (as test crates) during `cargo test`.
Why not

```rust
ex.filestem_str().map(|name| {
    TomlTarget {
        name: name.to_string(),
        ...
    }
})
```
?