wasmtime: Initial, partial support for externref

This is enough to get an `externref -> externref` identity function
passing.

However, `externref`s that are dropped by compiled Wasm code are (safely)
leaked. Follow up work will leverage cranelift's stack maps to resolve this
issue.

crates/c-api/macros/src/lib.rs

@@ -96,7 +96,7 @@ pub fn declare_ref(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
 
         #[no_mangle]
         pub extern fn #as_ref(a: &#ty) -> Box<crate::wasm_ref_t> {
-            let r = a.externref();
+            let r = Some(a.externref());
             Box::new(crate::wasm_ref_t { r })
         }