Update to the next version of the witx crate

This commit updates to the 0.9 version of the witx crate implemented in
WebAssembly/wasi#395. This new version drastically changes code
generation and how we interface with the crate. The intention is to
abstract the code generation aspects and allow code generators to
implement much more low-level instructions to enable more flexible APIs
in the future. Additionally a bunch of `*.witx` files were updated in
the WASI repository.

It's worth pointing out, however, that `wasi-common` does not change as
a result of this change. The shape of the APIs that we need to implement
are effectively the same and the only difference is that the shim
functions generated by wiggle are a bit different.

crates/wiggle/generate/src/module_trait.rs

@@ -7,17 +7,9 @@ use crate::names::Names;
 use witx::Module;
 
 pub fn passed_by_reference(ty: &witx::Type) -> bool {
-    let passed_by = match ty.passed_by() {
-        witx::TypePassedBy::Value { .. } => false,
-        witx::TypePassedBy::Pointer { .. } | witx::TypePassedBy::PointerLengthPair { .. } => true,
-    };
     match ty {
-        witx::Type::Builtin(b) => match &*b {
-            witx::BuiltinType::String => true,
-            _ => passed_by,
-        },
-        witx::Type::Pointer(_) | witx::Type::ConstPointer(_) | witx::Type::Array(_) => true,
-        _ => passed_by,
+        witx::Type::Pointer(_) | witx::Type::ConstPointer(_) | witx::Type::List(_) => true,
+        _ => false,
     }
 }
 
@@ -49,28 +41,36 @@ pub fn define_module_trait(names: &Names, m: &Module, errxform: &ErrorTransform)
             quote!(#arg_name: #arg_type)
         });
 
-        let result = if !f.noreturn {
-            let rets = f
-                .results
-                .iter()
-                .skip(1)
-                .map(|ret| names.type_ref(&ret.tref, lifetime.clone()));
-            let err = f
-                .results
-                .get(0)
-                .map(|err_result| {
-                    if let Some(custom_err) = errxform.for_abi_error(&err_result.tref) {
-                        let tn = custom_err.typename();
-                        quote!(super::#tn)
-                    } else {
-                        names.type_ref(&err_result.tref, lifetime.clone())
-                    }
-                })
-                .unwrap_or(quote!(()));
-            quote!( Result<(#(#rets),*), #err> )
-        } else {
-            let rt = names.runtime_mod();
-            quote!(#rt::Trap)
+        let rt = names.runtime_mod();
+        let result = match f.results.len() {
+            0 if f.noreturn => quote!(#rt::Trap),
+            0 => quote!(()),
+            1 => {
+                let (ok, err) = match &**f.results[0].tref.type_() {
+                    witx::Type::Variant(v) => match v.as_expected() {
+                        Some(p) => p,
+                        None => unimplemented!("anonymous variant ref {:?}", v),
+                    },
+                    _ => unimplemented!(),
+                };
+
+                let ok = match ok {
+                    Some(ty) => names.type_ref(ty, lifetime.clone()),
+                    None => quote!(()),
+                };
+                let err = match err {
+                    Some(ty) => match errxform.for_abi_error(ty) {
+                        Some(custom) => {
+                            let tn = custom.typename();
+                            quote!(super::#tn)
+                        }
+                        None => names.type_ref(ty, lifetime.clone()),
+                    },
+                    None => quote!(()),
+                };
+                quote!(Result<#ok, #err>)
+            }
+            _ => unimplemented!(),
         };
 
         if is_anonymous {