Import js macro using `use` requires importing _js_impl as well

[Dabo Ross]

As part of transition to rust 2018 idioms in one of my projects, I ran into an issue with macro imports in stdweb. Particularly, the following code fails to compile:

use stdweb::js;

fn main() {
    js! {
        console.log("Hello, world!");
    }
}

With the following error:

error: cannot find macro `_js_impl!` in this scope
 --> src/main.rs:4:5
  |
4 | /     js! {
5 | |         console.log("Hello, world!");
6 | |     }
  | |_____^
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

Importing the macro using #[macro_use] still works, as does

use stdweb::{_js_impl, js};

fn main() {
    js! {
        console.log("Hello, world!");
    }
}

The edition guide's macro changes section documents this error, as well as two ways to fix it. Since the minimum rust version for this crate is 1.33.0, I think using $crate:: to reference local macros should be reasonable?

Edit: realized the minimum rust version of stdweb is already 1.33.0