jextract: Support Foundation.Decimal

[tkrajacic]

We should be able to bridge it to BigDecimal probably?

[marcprux]

Should this be an issue for https://github.com/swiftlang/swift-java rather than https://github.com/swiftlang/swift-java-jni-core? If so, perhaps @ktoso can help relocate it.

Regardless, this is an interesting case. Foundation.Decimal and java.math.BigDecimal are broadly similar, but have quite different implementation details. For example java.math.BigDecimal is truly arbitrary-precision, but Foundation.Decimal is a fixed-size 160-bit struct with around 38 digits of precision. Further, java.math.BigDecimal("1.0") != java.math.BigDecimal("1.00"), but Foundation.Decimal would see those as equal.

So for some cases (e.g., "I just want a number bigger than a double"), bridging them might make sense. But for other cases it could be dangerous to assume that the two types are perfectly equivalent.

[ktoso]

Yeah this should be in swift-java, I'll move the issue.

Generally, swift-java can ONLY "convert" it a mapping is exactly 1:1, Foundation types are generally entwined with some history and objc-weirdness so they rarely match.

In general, we should just import BigDecimal "as" a swift-java wrapped BigDecimal and offer some to... converters