Support [AllowResizable], [AllowShared], SharedArrayBuffer, and AllowSharedBufferSource

[MattiasBuelens]

• SharedArrayBuffer is now a separate IDL type, and [AllowShared] BufferSource has been replaced with a new AllowSharedBufferSource typedef.
  • See: Give SharedArrayBuffer a dedicated type whatwg/webidl#1311
• Buffer source types can now have extended attributes such as [AllowResizable] and [AllowShared], which need to be checked by webidl-conversions. Thus, we now generate the appropriate conversion calls.
  • See: Integrate with the resizable buffers proposal whatwg/webidl#982 and Add allowResizable to (Shared)ArrayBuffer conversions webidl-conversions#51.
• Fix unions with callback functions and callback interfaces.
• Simplify generated code for object type in unions.
  • This also fixes the crash reported in Avoid generating broken code when the object type is in a union #231.

[MattiasBuelens]

@domenic Could you assign some reviewer(s) to this PR? I can't seem to do it myself.

[domenic]

I'll still work on jsdom stuff, don't worry. I plan to review this tomorrow :)

[domenic]

I appreciate the separate commits; it made reviewing this easier. If you're up for some rebase-fu, here are some minor notes:

• "Rename operations" -> "Rename test operations for clarity"
• Merged "Fix object type handling" and "Test object inside a union", and expand on the fix (maybe with reference to Avoid generating broken code when the object type is in a union #231).
• Merge "Add AllowSharedBufferSource" into "Support [AllowShared] on all ArrayBufferView types"
• Edit the commit message for "Generate conversions for ArrayBufferView types" to explain why we're doing this now when we didn't before.

Or, if you prefer, we can just do a big squash-and-merge with a sufficiently-detailed commit message.

[domenic]

Awesome stuff, thank you!

The repeated conversions for unions with primitives are a bit strange, but I see the follow the spec exactly. (I.e. https://webidl.spec.whatwg.org/#js-union steps 13.1 + 17.)

Probably some optimizations are possible for simple unions. But I think the right tactic there would be to work on the more general project of slimming down the generated code, driven by analyzing all of jsdom to see what are the worst offenders weighted by web platform prevalence.