Cannot use JS strings without copying them into wasm memory

[fitzgen]

This doesn't work, but it should:

#[wasm_bindgen]
extern {
    #[wasm_bindgen(js_name = String)]
    pub extern type JsString;

    pub fn slice(this: &JsString, start: u32, end: u32) -> JsString;
}

This is blocking all the String related bindings in #275.

I haven't dug in very far, but I think the issue is that strings are special cased by wasm-bindgen to always copy in/out of wasm memory, and so we can't reference them as an opaque extern type.

[kzvi]

Aside from js_name not working, this works for me. Here is an example test case of it working (see build.sh): https://github.com/kzvi/wbg_test

[kzvi]

This line in crates/backend/src/ast.rs makes the assumption that the literal type name of a method in Rust is going to be the same as the type name of a method in Javascript. This is why this is breaking.

In order to fix this, either:

• Every method declaration needs to be annotated with the JS type name, e.g. #[wasm_bindgen(method, js_class = String)]
• The code in the "backend" crate would need to have some mutable state to keep track of which class methods are being defined for.

[ohanar]

I believe this was resolved in #295.