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/environ/src/cranelift.rs

@@ -303,7 +303,7 @@ fn compile(env: CompileEnv<'_>) -> Result<ModuleCacheDataTupleType, CompileError
             let func_index = env.local.func_index(*i);
             let mut context = Context::new();
             context.func.name = get_func_name(func_index);
-            context.func.signature = env.local.func_signature(func_index).clone();
+            context.func.signature = env.local.native_func_signature(func_index).clone();
             if env.tunables.debug_info {
                 context.func.collect_debug_info();
             }