Fix resolution of `wasm_bindgen` in `js!` macro

[alvinhochun]

This fixes an issue that, when a dependent crate depends on stdweb for the use of the js! macro but does not specify the dependency wasm_bindgen, attempting to build for the target wasm32-unknown-unknown (or using wasm-pack and friends) will yield the following error:

error[E0433]: failed to resolve: use of undeclared type or module `wasm_bindgen`

---

Steps to reproduce/verify:

1. Apply the following change:

diff --git a/examples/Cargo.toml b/examples/Cargo.toml
index 6cc4623..129d50e 100644
--- a/examples/Cargo.toml
+++ b/examples/Cargo.toml
@@ -1,2 +1,5 @@
 [workspace]
 members = ["canvas", "drag", "echo", "futures", "gamepad", "hasher", "minimal", "todomvc", "webgl", "wasm-bindgen-minimal"]
+
+[patch.crates-io]
+stdweb = { path = ".." }
diff --git a/examples/wasm-bindgen-minimal/Cargo.toml b/examples/wasm-bindgen-minimal/Cargo.toml
index 848c70e..a29056b 100644
--- a/examples/wasm-bindgen-minimal/Cargo.toml
+++ b/examples/wasm-bindgen-minimal/Cargo.toml
@@ -10,3 +10,4 @@ crate-type = ["cdylib"]
 [dependencies]
 wasm-bindgen = "0.2"
 stdweb = { path = "../.." }
+instant = { version = "=0.1.2", features = [ "stdweb" ]}
diff --git a/examples/wasm-bindgen-minimal/src/lib.rs b/examples/wasm-bindgen-minimal/src/lib.rs
index 70c7517..bad6967 100644
--- a/examples/wasm-bindgen-minimal/src/lib.rs
+++ b/examples/wasm-bindgen-minimal/src/lib.rs
@@ -10,5 +10,7 @@ pub fn main() -> Result<(), JsValue> {
         alert( @{message} );
     }
 
+    instant::Instant::now();
+
     Ok(())
 }

2. From the directory examples/wasm-bindgen-minimal/, execute the command cargo build --lib --target wasm32-unknown-unknown.