Use web_sys::Url for IDN conversion on wasm32-unknown-unknown

[micolous]

This pull request is based on #886. Merging this PR will also merge #886.

This removes the dependency on idna on wasm32-unknown-unknown, replacing it with a small shim which calls out to web_sys::Url.

With a small yew demo app (which calls Url::parse()), this reduces binary size by 228 KiB on a release build and by 425 KiB on a debug build. These savings should go a lot of the way towards addressing #557, while retaining IDN support.

API changes

The deprecations could be split out from this, I've included them as related work.

• Deprecate quirks::domain_to_unicode(): Remove URL.domainToASCII and URL.domainToUnicode whatwg/url#63

  Removed on wasm32-unknown-unknown because browsers never implemented it.

• Deprecate Origin::unicode_serialization(): Mark Origin::unicode_serialization as deprecated? #881

  Removed on wasm32-unknown-unknown because it requires domain_to_unicode.

• Deprecate quirks::domain_to_ascii(): Remove URL.domainToASCII and URL.domainToUnicode whatwg/url#63

  Kept on wasm32-unknown-unknown, because its implementation does not depend on browser functionality.

Alternatives considered

• Downstream libraries could adopt web_sys::Url on wasm32-unknown-unknown.

  This is API-breaking, and complicated to handle, for reasons I've covered in WASM file size #557 and Use web_sys::Url instead of url::Url on wasm32 targets seanmonstar/reqwest#2050.

  There are similar pains discussed in Use url::Url instead of http::Uri sagebind/isahc#382 for a http::Uri to url::Url migration.

• Shimming out every single piece of functionality to JavaScript's Url type (via web_sys::Url) on wasm32-unknown-unknown.

  This would probably make things smaller again, but there are enough subtle API differences to make this a problem. For example, url::Url::host() returns a url::Host, but web_sys::Url::hostname() returns a String.

• Shimming in idna.

  Not all the functionality required is supported in web APIs (domain_to_unicode), and so it needs changes to Url anyway.

  This also allows one to continue using the idna crate directly on wasm32-unknown-unknown with its normalisation database.

Known issues

• There are a few extra test failures (see expected_failures_{chromium,firefox,safari}.txt) – I'm not certain which of these are browser bugs or url::Host bugs.

  In any case, I don't think I can fix them, and I don't know how much one should care.

• I haven't tried this on redox or wasi targets – they don't appear to be covered by CI, and so I've left them as-is.

  It may be possible to get similar binary size savings on those targets, but I'm keeping them out of scope for now.

[micolous]

While this is based on #886, I've marked it as "ready for review", as this isn't really a work in progress – it's just blocked.

[codecov]

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Additional details and impacted files

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