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/wast/src/spectest.rs

@@ -35,7 +35,7 @@ pub fn link_spectest(linker: &mut Linker) -> Result<()> {
     linker.define("spectest", "global_f64", g)?;
 
     let ty = TableType::new(ValType::FuncRef, Limits::new(10, Some(20)));
-    let table = Table::new(linker.store(), ty, Val::ExternRef(ExternRef::Null))?;
+    let table = Table::new(linker.store(), ty, Val::ExternRef(None))?;
     linker.define("spectest", "table", table)?;
 
     let ty = MemoryType::new(Limits::new(1, Some(2)));