Install ExecJS
Add to_javascript denotational semantics
Use JavaScript-friendly values as test doubles in Simple variable spec

We can't round-trip an instance of RSpec::Mocks::Mock to JavaScript and
back, so picking arbitrary JSON-compatible values will have to do.
Add JavaScript support to Simple denotation matchers
Add JavaScript denotational specs
Add remaining specs for JavaScript denotational semantics
Support round-tripped Ruby hashes in denotational semantics matcher