Don't rely on the target directory being named 'target'

[fbeutel]

Currently, paths::target_dir() tries to find the target directory by looking at the name.
This has two problems:

1. The user might have specified a different target directory by using the Cargo config option target-dir (https://doc.rust-lang.org/cargo/reference/config.html)
2. The target directory might be called target, but actually be a symlink to something else that isn't called target - and canonicalize resolves symlinks.

In both cases, paths::target_dir() fails.

What is the rationale for not putting all the generated code etc. in the OUT_DIR or a subdirectory thereof directly? If the latter is acceptable, I'm happy to submit a pull request changing the behavior...

[dtolnay]

I would appreciate a PR to improve this.

[dtolnay]

Fixed in 0.4.0.