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/src/extern.rs

@@ -18,12 +18,12 @@ pub(crate) enum ExternHost {
 }
 
 impl wasm_extern_t {
-    fn externref(&self) -> wasmtime::ExternRef {
+    pub(crate) fn externref(&self) -> wasmtime::ExternRef {
         match &self.which {
-            ExternHost::Func(f) => f.externref(),
-            ExternHost::Global(f) => f.externref(),
-            ExternHost::Memory(f) => f.externref(),
-            ExternHost::Table(f) => f.externref(),
+            ExternHost::Func(f) => f.clone().into(),
+            ExternHost::Global(f) => f.clone().into(),
+            ExternHost::Memory(f) => f.clone().into(),
+            ExternHost::Table(f) => f.clone().into(),
         }
     }
 }