Re-add --rust-target to replace --unstable-rust

Instead of specifying whether or not to use stable, specify the Rust
release to support (one of several stable/beta releases or nightly).
The `--unstable-rust` option is still accepted and implies nightly.

The definitions of `RustTarget` and `RustFeatures` are created with
macros.

For each test that uses unions, there is a version that uses the latest
stable and 1.0.

This also fixes the bug where unions were generated with non-Copy
fields.

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