Leaking outside of optimized functions not supported

[NTillmann]

This crashes Prepack. The issue that binding assignments don't have to happen in optimized functions.

let g = __abstract("function", "(obj)");
global.result = __evaluatePureFunction(() => {
    let x = 23;
    function incrementX() {
        x = x + 42;
    }
    global.__optimize && __optimize(incrementX);
    g(incrementX);
    return x;
});

==>

Invariant Violation:
This is likely a bug in Prepack, not your code. Feel free to open an issue on GitHub.
    at invariant (/Users/nikolait/git/prepack/lib/invariant.js:24:15)
    at Object.visitBindingAssignment (/Users/nikolait/git/prepack/lib/serializer/ResidualHeapVisitor.js:1182:34)
    at BindingAssignmentEntry.visit (/Users/nikolait/git/prepack/lib/utils/generator.js:358:26)
    at visitFn (/Users/nikolait/git/prepack/lib/utils/generator.js:818:46)
    at Generator.visit (/Users/nikolait/git/prepack/lib/utils/generator.js:826:7)
    at _withScope (/Users/nikolait/git/prepack/lib/serializer/ResidualHeapVisitor.js:1200:17)
    at ResidualHeapVisitor._withScope (/Users/nikolait/git/prepack/lib/serializer/ResidualHeapVisitor.js:208:7)
    at ResidualHeapVisitor.visitGenerator (/Users/nikolait/git/prepack/lib/serializer/ResidualHeapVisitor.js:1199:10)
    at ResidualHeapVisitor.visitRoots (/Users/nikolait/git/prepack/lib/serializer/ResidualHeapVisitor.js:1248:10)
    at statistics.deepTraversal.measure (/Users/nikolait/git/prepack/lib/serializer/serializer.js:204:70)