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.

tests/all/traps.rs

@@ -341,7 +341,7 @@ fn mismatched_arguments() -> Result<()> {
     );
     assert_eq!(
         func.call(&[Val::F32(0)]).unwrap_err().to_string(),
-        "argument type mismatch",
+        "argument type mismatch: found f32 but expected i32",
     );
     assert_eq!(
         func.call(&[Val::I32(0), Val::I32(1)])