Support formContext in conditions

[timkindberg]

I'm trying to make a PR to add support for the formContext in the rule conditions. Here is the new test I've added to your codebase. It's proving to be much more difficult (or rather, hacky) than I expected.

test("FormConditionals - onChange called with corrected schema when formContext changes", () => {
  const changed = sinon.spy(() => {});
  const RULES = [
    {
      conditions: {
        'formContext.foo': "empty",
      },
      event: {
        type: "remove",
        params: {
          field: ["lastName", "name"],
        },
      },
    },
  ];
  const props = { schema, uiSchema: {}, rules: RULES, Engine };
  const formContext = { foo: 1 };
  const wrapper = mount(
    <FormWithConditionals {...props} formData={{ firstName: "A" }} formContext={formContext} onChange={changed} />
  );

  wrapper.setProps({ formContext: { foo: undefined } });

  return new Promise(resolve => setTimeout(resolve, 500)).then(() => {
    const expSchema = {
      type: "object",
      properties: {
        firstName: { type: "string" },
      },
    };

    expect(changed.called).toEqual(true);
    expect(changed.calledOnce).toEqual(true);
    expect(changed.getCall(0).args[0].schema).toEqual(expSchema);
  });
});

Any chance you could give me your high level two cents on how you might implement this and I could cross check it with what I have? I'll open a WIP PR so you can see where I'm currently at.