Tracking issue for union improvements

[carols10cents]

Rust tracking issues: rust-lang/rust#32836 and rust-lang/rust#55149 look relevant

Place that needs to be updated: https://github.com/rust-lang-nursery/edition-guide/blame/68c458b9b68edced39aebea60e672a22d4d50edf/src/rust-2018/data-types/union-for-an-unsafe-form-of-enum.md#L58-L60

There’s still more improvements to come. For now, unions can only include Copy types and may not implement Drop. We expect to lift these restrictions in the future.

[ehuss]

Closing per #232, this section has been removed. For now, I think the only documentation is in the reference, which should get updated whenever more work on unions is done.