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/table.rs

@@ -4,9 +4,9 @@ use wasmtime::*;
 fn get_none() {
     let store = Store::default();
     let ty = TableType::new(ValType::FuncRef, Limits::new(1, None));
-    let table = Table::new(&store, ty, Val::ExternRef(ExternRef::Null)).unwrap();
+    let table = Table::new(&store, ty, Val::ExternRef(None)).unwrap();
     match table.get(0) {
-        Some(Val::ExternRef(ExternRef::Null)) => {}
+        Some(Val::ExternRef(None)) => {}
         _ => panic!(),
     }
     assert!(table.get(1).is_none());