wiggle: add support for async traits; ABI func is now generic of ctx

* ctx parameter no longer accepted by wiggle::from_witx macro.
* optional async_ parameter specifies which functions are async.
* re-export async_trait::async_trait, so users don't have to take a dep.

crates/wiggle/generate/src/names.rs

@@ -7,20 +7,12 @@ use witx::{BuiltinType, Id, Type, TypeRef, WasmType};
 use crate::{lifetimes::LifetimeExt, UserErrorType};
 
 pub struct Names {
-    ctx_type: Ident,
     runtime_mod: TokenStream,
 }
 
 impl Names {
-    pub fn new(ctx_type: &Ident, runtime_mod: TokenStream) -> Names {
-        Names {
-            ctx_type: ctx_type.clone(),
-            runtime_mod,
-        }
-    }
-
-    pub fn ctx_type(&self) -> Ident {
-        self.ctx_type.clone()
+    pub fn new(runtime_mod: TokenStream) -> Names {
+        Names { runtime_mod }
     }
 
     pub fn runtime_mod(&self) -> TokenStream {