Wasm arrays as arguments and return types

[ross-weir]

Changes

This PR adds the following:

• Allows downcasting a JsValue into a rust struct that is wasm binded (required for the below 2 array ops)
• Allow rust accepting array of wasm structs from JS
• Allow returning array of structs from rust to JS

Related wasm-bindgen issues:

• Downcast a JsValue to an exported struct rustwasm/wasm-bindgen#2231
• Allow returning Vec<T> rustwasm/wasm-bindgen#111

The problem with using JSON / serde for handling passing/receiving arrays is that deser/ser converts to JS objects instead of classes so library consumers can't use class methods without converting to or from JSON first. With the derives added in this PR this is no longer needed

Simple example not using arrays but to describe the problem with JSON:

Rust

#[wasm_bindgen]
#[derive(Serialize, Deserialize)]
pub struct MyType(usize);

#[wasm_bindgen]
impl MyType {
    pub fn new(&self, n: usize) -> MyType {
        MyType(n)
    }

    pub fn inner_num(&self) -> usize {
        self.0
    }

    pub fn to_json(&self) -> Result<JsValue, serde_wasm_bindgen::Error> {
        serde_wasm_bindgen::to_value(&self.0)
    }
}

JS

const myType = MyType.new(5);
const myTypeJson = myType.to_json();

myType.inner_num(); // 5
myTypeJson.inner_num(); // doesn't work, json object doesn't have the func

const jsonBackToMyType = MyType.from_json(myTypeJson);
jsonBackToMyType.inner_num(); // 5

The hope is one day wasm_bindgen supports vecs of structs so we can remove this 😄

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[ross-weir]

TODO: remove the unwraps in this func

[greenhat]

This looks awesome! It improves the DX enormously and removes so much boilerplate. Great job!