Implement JsCast for all imported types

This commit implements the `JsCast` trait automatically for all imported types
in `#[wasm_bindgen] extern { ... }` blocks. The main change here was to generate
an `instanceof` shim for all imported types in case it's needed.

All imported types now also implement `AsRef<JsValue>` and `AsMut<JsValue>`

crates/shared/src/lib.rs

@@ -3,7 +3,7 @@
 #[macro_use]
 extern crate serde_derive;
 
-pub const SCHEMA_VERSION: &str = "7";
+pub const SCHEMA_VERSION: &str = "8";
 
 #[derive(Deserialize)]
 pub struct ProgramOnlySchema {
@@ -81,7 +81,10 @@ pub struct ImportStatic {
 }
 
 #[derive(Deserialize, Serialize)]
-pub struct ImportType {}
+pub struct ImportType {
+    pub name: String,
+    pub instanceof_shim: String,
+}
 
 #[derive(Deserialize, Serialize)]
 pub struct ImportEnum {}