Remove usage of wasm_import_module feature

This is now stabilized! Also tweak usage of it to the stable version.

tests/all/typescript.rs

@@ -7,7 +7,7 @@ fn works() {
         .file(
             "src/lib.rs",
             r#"
-                #![feature(use_extern_macros, wasm_import_module)]
+                #![feature(use_extern_macros)]
 
                 extern crate wasm_bindgen;