Treat String, ConsString, Boolean, and Double as value types

[szegedi]

I've been helping some folks that rely heavily in continuations comparisons in their product, and they had weird failures to compare continuations as equal. Turns out it's mostly due to differences in deserialized objects not having their constructor names etc. interned. The solution ultimately is to (correctly) consider JavaScript strings, booleans, and numbers to not have identity but treat them as purely value types, in line with the JS language semantics.

Closes #1192

[tonygermano]

I think internally any java.lang.Number can represent a javascript primitive number, with the exception of BigInteger, which is a javascript bigint.

[szegedi]

Are you sure about that? Even otherwise integer literals will end up being represented as doubles in Rhino, e.g.

import org.mozilla.javascript.Context;

public class TestNumbers {
    public static void main(String[] args) {
        try (var cx = Context.enter()) {
            cx.setOptimizationLevel(9);
            var scope = cx.initStandardObjects();
            var result = cx.evaluateString(scope, "1", "", 1, null);
            System.out.println(result.getClass().getName());
        }
    }
}

prints:

java.lang.Double

(Notice I used the most aggressive optimization level.) I could add additional types that we certainly know the engine itself produces internally, but I wouldn't blanket treat all Number subclasses especially since there can be user-defined subclasses too.

[szegedi]

Okay, I modified the code to use by-value comparison for all Java primitive type wrappers.

[szegedi]

This is the closest we can get to a Set.of in a pre-9 Java; currently Rhino still compiles for 8.

[rbri]

Thanks....