Make rustc an opt-in feature, use tester for test crate

[Munksgaard]

Fixes #135

[Munksgaard]

This may break some users, so I suggest we bump either the minor or major version before releasing.

[laumann]

At the very least, it should be a minor version (in a separate commit).

If it's only a minor release, I think you should also leave in the stable feature flag (but have it do nothing), so as not to break users of that flag. I'm not sure what to do with users that do not use stable.

[Munksgaard]

If it's only a minor release, I think you should also leave in the stable feature flag (but have it do nothing), so as not to break users of that flag. I'm not sure what to do with users that do not use stable.

Good point, I've gone ahead and done that.