Upgrade to witx 0.8.0 with tagged unions (#921)

* 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the bindgen tests became out-of-date with the latest changes to the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

crates/wasi-common/wig/Cargo.toml

@@ -17,7 +17,7 @@ proc-macro = true
 quote = "1.0.2"
 proc-macro2 = "1.0.6"
 heck = "0.3.1"
-witx = "0.7.0"
+witx = { path = "WASI/tools/witx" }
 
 [badges]
 maintenance = { status = "actively-developed" }