Update check-core to allow for overriding constants and lookups at runtime

Now that we have support for overriding constants and/or lookups at runtime in generated models and the runtime packages (see #470 and #472), we need to update the check-core APIs to allow for passing overrides to a check bundle.  We won't be using this support right away in the model-check UI, but at CI we have some internal tools that will use these new options.
