breaking change: improve `as_`/`is_` helpers (#527)

* breaking change: improve `as_`/`is_` helpers

- `as_` helpers for unions now return `Result` to clearly indicate what the actual value was
- Union variants now implement `TryFrom` for the parent union
- Error types now generate `is_` helpers

* bump version to v0.0.9-alpha

* Remove unsound TryFrom

* fix unit test and cargo fmt failure

* Delete unused import