Implement #[wasm_bindgen(extends = ...)]

This commit implements the `extends` attribute for `#[wasm_bindgen]` to
statically draw the inheritance hierarchy in the generated bindings, generating
appropriate `AsRef`, `AsMut`, and `From` implementations.

guide/src/reference/attributes/on-js-imports/extends.md

@@ -0,0 +1,49 @@
+# `extends = Class`
+
+The `extends` attribute can be used to say that an imported type extends (in the
+JS class hierarchy sense) another type. This will generate `AsRef`, `AsMut`, and
+`From` impls for converting a type into another given that we statically know
+the inheritance hierarchy:
+
+```rust
+#[wasm_bindgen]
+extern {
+    type Foo;
+
+    #[wasm_bindgen(extends = Foo)]
+    type Bar;
+}
+
+let x: &Bar = ...;
+let y: &Foo = x.as_ref(); // zero cost cast
+```
+
+The trait implementations generated for the above block are:
+
+```rust
+impl From<Bar> for Foo { ... }
+impl AsRef<Foo> for Bar { ... }
+impl AsMut<Foo> for Bar { ... }
+```
+
+
+The `extends = ...` attribute can be specified multiple times for longer
+inheritance chains, and `AsRef` and such impls will be generated for each of
+the types.
+
+```rust
+#[wasm_bindgen]
+extern {
+    type Foo;
+
+    #[wasm_bindgen(extends = Foo)]
+    type Bar;
+
+    #[wasm_bindgen(extends = Foo, extends = Bar)]
+    type Baz;
+}
+
+let x: &Baz = ...;
+let y1: &Bar = x.as_ref();
+let y2: &Foo = x.as_ref();
+```