Implement support for js_class on exported types

Allow defining types which have different names in Rust than they have
in JS! (just like can be done with imported types)

Closes #1010

guide/src/reference/attributes/on-rust-exports/js_class.md

@@ -0,0 +1,28 @@
+# `js_class = Blah`
+
+The `js_class` attribute is used to indicate that all the methods inside an
+`impl` block should be attached to the specified JS class instead of inferring
+it from the self type in the `impl` block. The `js_class` attribute is most
+frequently paired with [the `js_name` attribute](js_name.html) on structs:
+
+```rust
+#[wasm_bindgen(js_name = Foo)]
+pub struct JsFoo { /* ... */ }
+
+#[wasm_bindgen(js_class = Foo)]
+impl JsFoo {
+    #[wasm_bindgen(constructor)]
+    pub fn new() -> JsFoo { /* ... */ }
+
+    pub fn foo(&self) { /* ... */ }
+}
+```
+
+which is accessed like:
+
+```rust
+import { Foo } from './my_module';
+
+const x = new Foo();
+x.foo();
+```

guide/src/reference/attributes/on-rust-exports/js_name.md

@@ -22,3 +22,33 @@ import { doTheThing } from './my_module';
 const x = doTheThing();
 console.log(x);
 ```
+
+Like imports, `js_name` can also be used to rename types exported to JS:
+
+```rust
+#[wasm_bindgen(js_name = Foo)]
+pub struct JsFoo {
+    // ..
+}
+```
+
+to be accessed like:
+
+```js
+import { Foo } from './my_module';
+
+// ...
+```
+
+Note that attaching methods to the JS class `Foo` should be done via the
+[`js_class` attribute](js_class.html):
+
+```rust
+#[wasm_bindgen(js_name = Foo)]
+pub struct JsFoo { /* ... */ }
+
+#[wasm_bindgen(js_class = Foo)]
+impl JsFoo {
+    // ...
+}
+```