WASM backend

[yoshuawuyts]

WIP wasm backend. Should work out of the box. A lot of changes here include changing our middleware and backends to "just work" in the browser.

Currently running into some problems where we can't use await and send JsObject's between threads, possibly related to rustwasm/wasm-bindgen#1695. Should probably spend more time figuring it out.

https://github.com/yoshuawuyts/surf/blob/b756107dfce8553a10c1bc1220ef627244b56b8a/src/http_client/wasm.rs#L33-L44

[yoshuawuyts]

Okay, into a bit of a corner here. Because:

1. we can't send JS refs over Send bounds.
2. threads aren't supported in WASM yet
3. we shouldn't change the interface of our code just for WASM.

So I think we'll have to do a manual unsafe { impl Send for MyType } because threads don't exist in the browser yet, so we can't ever have multi-threaded problems anyway lmao.

[ghost]

I wonder what's the reason for JS refs not being sendable between threads. Do you think that's just an overly conservative measure in Rust's wasm bindings? Or is there a deeper reason truly preventing those values from being sendable?

[yoshuawuyts]

@stjepang Only JS vectors and strings can be sent over thread bounds (web workers). Rich data types such as DOM nodes have to be serialized / deserialized first. The Send bound is very real in this case.

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This patch is good to go I think! We're using a whole lot of unwrap in the main code still tho, but I think that's just a matter of polishing. Going to open a separate issue for that.

[yoshuawuyts]

Okay, I kind of went off the rails here and started implementing parts of #26. In particular there's now an internal abstraction we could probably PR to gloo. I swear we're good for now, ahaha.

[yoshuawuyts]

Going to go ahead and merge this!