Add Allure facade as runtime API

[delatrie]

An API similar to Allure.java should be available to users.

Motivation

There are multiple API levels in Allure:

1. The low-level API: AllureLifecycle, AllureContext.
2. The mid-level API (now missing): a bunch of methods to manipulate the current test/fixture/step at runtime.
3. The high-level API: attributes

The lack of the mid-level API makes it hard to affect the Allure context dynamically (e.g., to apply a dynamically generated title to a test result). Using the low-level API for that is inconvenient and error-prone.

Additional info

CoreStepsHelper's methods should be deprecated in favor of the new ones.