BigInt support

[MaxDesiatov]

BigInt is supported in Safari 14 on both macOS Catalina/Big Sur and iOS 14, makes perfect sense to support it on our side too, especially to allow proper handling of Int64.

Need to be careful about browser compatibility though. Should JavaScriptKit just fatalError on older browsers when users attempt to use JSBigInt in their code?

Also a good case for a pitch for proper browser versioning in Swift, I'll open a separate toolchain issue for it.

[j-f1]

I was thinking about doing this earlier today, and was trying to figure out how Int64/UInt64 fit into this. Ideally we’d be able to access Big(U)Int64Array from Swift using the existing TypedArray infrastructure but I’m not sure how passing the value across the bridge would work — do we just add two more JSValue types for Int64 and UInt64?

[MaxDesiatov]

Sadly, BigInt64Array isn't supported even in Safari 14. ☹️ Maybe still worth implementing it, just need to be even more careful with runtime checks?

[kateinoigakukun]

BigInt is now supported on Safari, but the latest WebKit doesn't allow to have i64 on the signature of imported function.
For this reason, I had to polyfill wasi functions that have i64 on its interface. https://github.com/kateinoigakukun/swiftwasm-pad/tree/master/Frontend/Sources/i64_polyfill

See also: WebKit test case https://github.com/WebKit/webkit/blob/3a6be938ed0ede574ca96575dbe69f572269c819/JSTests/wasm/function-tests/function-import-return-value.js#L130-L184