* witx tagged unions: updates to wig to use new semantics
* wig: emit a `#variant: ()` union variant for empty variants
* wasi-common: translate to use tagged unions
* update to flattened layout of event struct
* wig: generate layout tests, and delete bindgen ones
the bindgen tests became out-of-date with the latest changes to the
representation of unions, and the re-jiggering of various struct
definitions that went along with it.
* wasi: point at master with tagged union PR merged
* fix event struct repr on windows
4460e569cf