Auto merge of #892 - tmfink:feature-832-custom-rust-target, r=emilio

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

Re-apply commit. Addresses #832

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.

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