Implement stubs for bindings used by the unit tests

[rocketnia]

In the browser-based REPL, running the (rat) unit tests gives one test failure with a very clear error message: "No filesystem."

We could do the same thing for other operations the unit tests use, such as java-new. Having some binding for java-new will then cause a few more files of unit tests to run, as per the conditional logic in src/arc/lib/unit-test.arc. (Like the other unit tests, these extra ones pass except for some expected failures, like when they attempt to use the java-... FFI operations.)