Implement stages

Stages are sort of "flat steps" with body following the call within current context (test or step body). End of context or another stage start should finish a stage.

**Proposed API:** `Allure::startStage(?string $name = null): void`. We cannot use annotations with this API, so we'll have to use `Allure::*` shortcuts to decorate stage.

**Problems:** As for now, shortcuts such as `Allure::parameter()` deal with "current test" context; to implement effective stages we'll have to make them work with "current test or step" context, changing "classic" behavior. Another way to solve this problem is to provide alternate shortcuts, either in `Allure::stepParameter()` or `AllureStep::parameter()` form. We can also change "classic" behavior but provide fallbacks like `Allure::testParameter()` for those who want to decorate test from within a step. And one more way is to return `StepContextInterface` from stage starting method.

Start with the proposed Allure::startStage(?string $name = null): void API and review how the existing Allure::* shortcuts resolve the current test context. Compare the listed alternatives for decorating stages and decide how test-versus-step context should behave. Done means stages close at context boundaries and the shortcut API behavior is specified and implemented.

Written by the indexing model from the issue text.