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the bindgen tests became out-of-date with the latest changes to the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

Cargo.lock

@@ -496,6 +496,12 @@ dependencies = [
  "synstructure",
 ]
 
+[[package]]
+name = "diff"
+version = "0.1.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0e25ea47919b1560c4e3b7fe0aaab9becf5b84a10325ddf7db0f0ba5e1026499"
+
 [[package]]
 name = "digest"
 version = "0.8.1"
@@ -2231,13 +2237,13 @@ dependencies = [
 
 [[package]]
 name = "witx"
-version = "0.7.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a6ee25990cb94f39b8d5637b8c7d9b9eaaed76795d95f45342a7ef3d2e574931"
+version = "0.8.0"
 dependencies = [
- "clap",
+ "anyhow",
+ "diff",
  "log",
  "pretty_env_logger",
+ "structopt",
  "thiserror",
  "wast 3.0.4",
 ]