Allow creating Vecs of and implement TryFrom<JsValue> for strings and exported Rust types

[Liamolucko]

This is a rebase of #2734 along with some fixes and tweaks. Sorry it took me so long to get to it!

This PR:

• Adds support for passing Vecs of Rust structs to/from JS.
• Adds support for passing Vec<String>s to/from JS.
• Implements TryFrom<JsValue> for Rust structs and strings.

The Vec impls are little inefficient, since they work by passing Vec<JsValue>s to/from JS like normal, and then translating them to the desired Vec with TryFrom / Into<JsValue>, which requires throwing away the first allocation and making a second one.

Fixes #111
Fixes #2452
Fixes #168
Fixes #2231

This also mostly supersedes #3088. That PR does support slightly more functionality (getting access to &Struct instead of forcing you to take ownership of it and invalidate JS's handle), but it also does so by exposing the internal Anchor type used by RefFromWasmAbi, which I'm not a huge fan of.

I haven't bumped the schema version, since the actual schema hasn't changed and so older versions of the CLI are mostly still compatible, although they do give this cryptic error message if you try and use the new TryFrom<JsValue> impl for Rust structs:

error: import of `__wbg_foo_unwrap` doesn't have an adapter listed

(where foo is the name of the struct.)

So maybe it's worth bumping it anyway, not sure.

[daxpedda]

I did a quick review and it LGTM.
Maybe I will find some time to make a proper review, but feel free to just merge if you are feeling confident about this.

[hamza1311]

Does this allow returning Vec<_> or such from externs? That's been a big limitation in my use of ducktor for complicated types

[Liamolucko]

Does this allow returning Vec<_> or such from externs? That's been a big limitation in my use of ducktor for complicated types

If it's a Vec of #[wasm_bindgen]'d structs or `Strings, yes.

[sinking-point]

Thank you @Liamolucko for sorting this out. It's so good to see my work being put to use.

[cschramm]

I think this only partially fixes #111 and #168 (depending on their scopes).

Is it expected that it does not work for returning Vecs from async functions? For both String and #[wasm_bindgen]d structs I get an error that From<Vec<_>> is not implemented for JsValue.

Minimal example:

#[wasm_bindgen]
pub async fn f() -> Vec<String> {
    unimplemented!()
}

I can imagine that it might be an easy target, though.

Side node: Passing Vecs to async fns works fine.

[aspect]

As this came into 0.2.88 it just broke all our framework bindings - #3685

TLDR; this implements a trait on types created by the developer who is using the wasm-bindgen framework, preventing the developer from creating his own TryFrom<JsValue> implementations, which can have many use cases.