[ruby/irb] Pass statements to Context#evaluate

(https://github.com/ruby/irb/pull/920)

This has a few benefits:

- We can keep hiding the evaluation logic inside the Context level, which
  has always been the convention until #824 was merged recently.
- Although not an official API, gems like `debug` and `mission_control-jobs`
  patch `Context#evaluate` to wrap their own logic around it. This implicit
  contract was broken after #824, and this change restores it.

In addition to the refactor, I also converted some context-level evaluation
tests into integration tests, which are more robust and easier to maintain.

https://github.com/ruby/irb/commit/b32aee4068

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