Having loom as dev-dependency incompatible with loom integration tests

[faern]

The main readme recommends adding loom as a dev-dependency. Then it goes on to mention having loom tests as integration tests:

For example, if the library includes a test file: tests/loom_my_struct.rs ...

This does not work. Because when building integration tests the main library is not built in #[cfg(test)] mode but rather in the same way cargo build builds it. So the dev-dependencies are not available. As such, the crate can't use loom::thread::spawn etc internally.

Would you recommend only doing loom tests in unit tests? This is what I have seen some of the libraries using loom doing. Or having loom as a normal, but optional, dependency under [dependencies]? Then anyone can activate them when they use the crate. May be good or bad. The bad part is that the crate exposes features normal users are not meant to use. The good part is If the crate itself implements synchronization primitives, then users of that crate can activate the loom feature if they want to loom test their stuff depending on the synchronization primitives in this library.

[carllerche]

Does this change work? #137

[faern]

That's better, thanks!